user/parlib/uthread.c

#include <ros/arch/membar.h>
#include <arch/atomic.h>
#include <parlib.h>
#include <vcore.h>
#include <uthread.h>
#include <event.h>
#include <stdlib.h>

/* Which operations we'll call for the 2LS.  Will change a bit with Lithe.  For
 * now, there are no defaults.  2LSs can override sched_ops. */
struct schedule_ops default_2ls_ops = {0};
struct schedule_ops *sched_ops __attribute__((weak)) = &default_2ls_ops;

__thread struct uthread *current_uthread = 0;
/* ev_q for all preempt messages (handled here to keep 2LSs from worrying
 * extensively about the details.  Will call out when necessary. */
struct event_queue *preempt_ev_q;

/* Helpers: */
#define UTH_TLSDESC_NOTLS (void*)(-1)
static inline bool __uthread_has_tls(struct uthread *uthread);
static int __uthread_allocate_tls(struct uthread *uthread);
static int __uthread_reinit_tls(struct uthread *uthread);
static void __uthread_free_tls(struct uthread *uthread);
static void __run_current_uthread_raw(void);

/* Block the calling uthread on sysc until it makes progress or is done */
static void __ros_mcp_syscall_blockon(struct syscall *sysc);

/* Helper, make the uthread code manage thread0.  This sets up uthread such
 * that the calling code and its TLS are tracked by the uthread struct, and
 * vcore0 thinks the uthread is running there.  Called only by slim_init (early
 * _S code) and lib_init.
 *
 * Whether or not uthreads have TLS, thread0 has TLS, given to it by glibc.
 * This TLS will get set whenever we use thread0, regardless of whether or not
 * we use TLS for uthreads in general.  glibc cares about this TLS and will use
 * it at exit.  We can't simply use that TLS for VC0 either, since we don't know
 * where thread0 will be running when the program ends. */
static void uthread_manage_thread0(struct uthread *uthread)
{
	assert(uthread);
	/* Save a pointer to thread0's tls region (the glibc one) into its tcb */
	uthread->tls_desc = get_tls_desc(0);
	/* Save a pointer to the uthread in its own TLS */
	current_uthread = uthread;
	/* Thread is currently running (it is 'us') */
	uthread->state = UT_RUNNING;
	/* utf/as doesn't represent the state of the uthread (we are running) */
	uthread->flags &= ~(UTHREAD_SAVED | UTHREAD_FPSAVED);
	/* need to track thread0 for TLS deallocation */
	uthread->flags |= UTHREAD_IS_THREAD0;
	/* Change temporarily to vcore0s tls region so we can save the newly created
	 * tcb into its current_uthread variable and then restore it.  One minor
	 * issue is that vcore0's transition-TLS isn't TLS_INITed yet.  Until it is
	 * (right before vcore_entry(), don't try and take the address of any of
	 * its TLS vars. */
	set_tls_desc(get_vcpd_tls_desc(0), 0);
	/* We might have a basic uthread already installed (from slim_init), so
	 * free it before installing the new one. */
	if (current_uthread)
		free(current_uthread);
	current_uthread = uthread;
	set_tls_desc(uthread->tls_desc, 0);
	__vcoreid = 0;	/* setting the uthread's TLS var */
	assert(!in_vcore_context());
}

/* The real 2LS calls this, passing in a uthread representing thread0.  When it
 * returns, you're in _M mode, still running thread0, on vcore0 */
void uthread_lib_init(struct uthread *uthread)
{
	init_once_racy(return);
	vcore_init();
	uthread_manage_thread0(uthread);
	/* Receive preemption events.  Note that this merely tells the kernel how to
	 * send the messages, and does not necessarily provide storage space for the
	 * messages.  What we're doing is saying that all PREEMPT and CHECK_MSGS
	 * events should be spammed to vcores that are running, preferring whatever
	 * the kernel thinks is appropriate.  And IPI them.
	 *
	 * It is critical that these are either SPAM_PUB or INDIR|FALLBACK, so that
	 * yielding vcores do not miss the preemption messages. */
	ev_handlers[EV_VCORE_PREEMPT] = handle_vc_preempt;
	ev_handlers[EV_CHECK_MSGS] = handle_vc_indir;
	preempt_ev_q = get_event_q();	/* small ev_q, mostly a vehicle for flags */
	preempt_ev_q->ev_flags = EVENT_IPI | EVENT_SPAM_PUBLIC | EVENT_VCORE_APPRO |
							 EVENT_VCORE_MUST_RUN;
	/* Tell the kernel to use the ev_q (it's settings) for the two types.  Note
	 * that we still have two separate handlers.  We just want the events
	 * delivered in the same way.  If we ever want to have a big_event_q with
	 * INDIRs, we could consider using separate ones. */
	register_kevent_q(preempt_ev_q, EV_VCORE_PREEMPT);
	register_kevent_q(preempt_ev_q, EV_CHECK_MSGS);
	printd("[user] registered %08p (flags %08p) for preempt messages\n",
	       preempt_ev_q, preempt_ev_q->ev_flags);
	/* Get ourselves into _M mode.  Could consider doing this elsewhere... */
	vcore_change_to_m();
}

/* Helper: tells the kernel our SCP is capable of going into vcore context on
 * vcore 0.  Pairs with k/s/process.c scp_is_vcctx_ready(). */
static void scp_vcctx_ready(void)
{
	struct preempt_data *vcpd = vcpd_of(0);
	long old_flags;
	/* the CAS is a bit overkill; keeping it around in case people use this
	 * code in other situations. */
	do {
		old_flags = atomic_read(&vcpd->flags);
		/* Spin if the kernel is mucking with the flags */
		while (old_flags & VC_K_LOCK)
			old_flags = atomic_read(&vcpd->flags);
	} while (!atomic_cas(&vcpd->flags, old_flags,
	                     old_flags & ~VC_SCP_NOVCCTX));
}

/* For both of these, VC ctx uses the usual TLS errno/errstr.  Uthreads use
 * their own storage.  Since we're called after manage_thread0, we should always
 * have current_uthread if we are not in vc ctx. */
static int *__ros_errno_loc(void)
{
	if (in_vcore_context())
		return __errno_location_tls();
	else
		return &current_uthread->err_no;
}

static char *__ros_errstr_loc(void)
{
	if (in_vcore_context())
		return __errstr_location_tls();
	else
		return current_uthread->err_str;
}

/* Slim-init - sets up basic uthreading for when we are in _S mode and before
 * we set up the 2LS.  Some apps may not have a 2LS and thus never do the full
 * vcore/2LS/uthread init. */
void uthread_slim_init(void)
{
	struct uthread *uthread;
	int ret = posix_memalign((void**)&uthread, __alignof__(struct uthread),
	                         sizeof(struct uthread));
	assert(!ret);
	memset(uthread, 0, sizeof(struct uthread));	/* aggressively 0 for bugs */
	/* TODO: consider a vcore_init_vc0 call. */
	vcore_init();
	uthread_manage_thread0(uthread);
	scp_vcctx_ready();
	init_posix_signals();
	/* change our blockon from glibc's internal one to the mcp one (which can
	 * handle SCPs too).  we must do this before switching to _M, or at least
	 * before blocking while an _M.  it's harmless (and probably saner) to do it
	 * earlier, so we do it as early as possible. */
	ros_syscall_blockon = __ros_mcp_syscall_blockon;
	/* Switch our errno/errstr functions to be uthread-aware.  See glibc's
	 * errno.c for more info. */
	ros_errno_loc = __ros_errno_loc;
	ros_errstr_loc = __ros_errstr_loc;
}

/* 2LSs shouldn't call uthread_vcore_entry directly */
void __attribute__((noreturn)) uthread_vcore_entry(void)
{
	uint32_t vcoreid = vcore_id();
	struct preempt_data *vcpd = vcpd_of(vcoreid);
	/* Should always have notifications disabled when coming in here. */
	assert(!notif_is_enabled(vcoreid));
	assert(in_vcore_context());
	/* If someone is stealing our uthread (from when we were preempted before),
	 * we can't touch our uthread.  But we might be the last vcore around, so
	 * we'll handle preemption events (spammed to our public mbox).
	 *
	 * It's important that we only check/handle one message per loop, otherwise
	 * we could get stuck in a ping-pong scenario with a recoverer (maybe). */
	while (atomic_read(&vcpd->flags) & VC_UTHREAD_STEALING) {
		/* Note we're handling INDIRs and other public messages while someone
		 * is stealing our uthread.  Remember that those event handlers cannot
		 * touch cur_uth, as it is "vcore business". */
		handle_one_mbox_msg(&vcpd->ev_mbox_public);
		cpu_relax();
	}
	/* If we have a current uthread that is DONT_MIGRATE, pop it real quick and
	 * let it disable notifs (like it wants to).  Other than dealing with
	 * preemption events (or other INDIRs), we shouldn't do anything in vc_ctx
	 * when we have a DONT_MIGRATE uthread. */
	if (current_uthread && (current_uthread->flags & UTHREAD_DONT_MIGRATE))
		__run_current_uthread_raw();
	/* Check and see if we wanted ourselves to handle a remote VCPD mbox.  Want
	 * to do this after we've handled STEALING and DONT_MIGRATE. */
	try_handle_remote_mbox();
	/* Otherwise, go about our usual vcore business (messages, etc). */
	handle_events(vcoreid);
	__check_preempt_pending(vcoreid);
	assert(in_vcore_context());	/* double check, in case an event changed it */
	/* Consider using the default_2ls_op for this, though it's a bit weird. */
	if (sched_ops->sched_entry) {
		sched_ops->sched_entry();
	} else if (current_uthread) {
		run_current_uthread();
	}
	/* 2LS sched_entry should never return */
	/* Either the 2LS sched_entry returned, run_cur_uth() returned, or we
	 * didn't have a current_uthread.  If we didn't have a 2LS op, we should be
	 * in _S mode and always have a current_uthread. */
	assert(0);
}

/* Does the uthread initialization of a uthread that the caller created.  Call
 * this whenever you are "starting over" with a thread. */
void uthread_init(struct uthread *new_thread, struct uth_thread_attr *attr)
{
	int ret;
	assert(new_thread);
	new_thread->state = UT_NOT_RUNNING;
	/* They should have zero'd the uthread.  Let's check critical things: */
	assert(!new_thread->flags && !new_thread->sysc);
	/* the utf holds the GP context of the uthread (set by the 2LS earlier).
	 * There is no FP context to be restored yet.  We only save the FPU when we
	 * were interrupted off a core. */
	new_thread->flags |= UTHREAD_SAVED;
	if (attr && attr->want_tls) {
		/* Get a TLS.  If we already have one, reallocate/refresh it */
		if (new_thread->tls_desc)
			ret = __uthread_reinit_tls(new_thread);
		else
			ret = __uthread_allocate_tls(new_thread);
		assert(!ret);
		uthread_set_tls_var(new_thread, current_uthread, new_thread);
	} else {
		new_thread->tls_desc = UTH_TLSDESC_NOTLS;
	}
}

/* This is a wrapper for the sched_ops thread_runnable, for use by functions
 * outside the main 2LS.  Do not put anything important in this, since the 2LSs
 * internally call their sched op.  This is to improve batch wakeups (barriers,
 * etc) */
void uthread_runnable(struct uthread *uthread)
{
	assert(sched_ops->thread_runnable);
	sched_ops->thread_runnable(uthread);
}

/* Informs the 2LS that its thread blocked, and it is not under the control of
 * the 2LS.  This is for informational purposes, and some semantic meaning
 * should be passed by flags (from uthread.h's UTH_EXT_BLK_xxx options).
 * Eventually, whoever calls this will call uthread_runnable(), giving the
 * thread back to the 2LS.
 *
 * If code outside the 2LS has blocked a thread (via uthread_yield) and ran its
 * own callback/yield_func instead of some 2LS code, that callback needs to
 * call this.
 *
 * AKA: obviously_a_uthread_has_blocked_in_lincoln_park() */
void uthread_has_blocked(struct uthread *uthread, int flags)
{
	if (sched_ops->thread_has_blocked)
		sched_ops->thread_has_blocked(uthread, flags);
}

/* Need to have this as a separate, non-inlined function since we clobber the
 * stack pointer before calling it, and don't want the compiler to play games
 * with my hart. */
static void __attribute__((noinline, noreturn))
__uthread_yield(void)
{
	struct uthread *uthread = current_uthread;
	assert(in_vcore_context());
	assert(!notif_is_enabled(vcore_id()));
	/* Note: we no longer care if the thread is exiting, the 2LS will call
	 * uthread_destroy() */
	uthread->flags &= ~UTHREAD_DONT_MIGRATE;
	uthread->state = UT_NOT_RUNNING;
	/* Do whatever the yielder wanted us to do */
	assert(uthread->yield_func);
	uthread->yield_func(uthread, uthread->yield_arg);
	/* Make sure you do not touch uthread after that func call */
	/* Leave the current vcore completely */
	/* TODO: if the yield func can return a failure, we can abort the yield */
	current_uthread = NULL;
	/* Go back to the entry point, where we can handle notifications or
	 * reschedule someone. */
	uthread_vcore_entry();
}

/* Calling thread yields for some reason.  Set 'save_state' if you want to ever
 * run the thread again.  Once in vcore context in __uthread_yield, yield_func
 * will get called with the uthread and yield_arg passed to it.  This way, you
 * can do whatever you want when you get into vcore context, which can be
 * thread_blockon_sysc, unlocking mutexes, joining, whatever.
 *
 * If you do *not* pass a 2LS sched op or other 2LS function as yield_func,
 * then you must also call uthread_has_blocked(flags), which will let the 2LS
 * know a thread blocked beyond its control (and why). */
void uthread_yield(bool save_state, void (*yield_func)(struct uthread*, void*),
                   void *yield_arg)
{
	struct uthread *uthread = current_uthread;
	volatile bool yielding = TRUE; /* signal to short circuit when restarting */
	assert(!in_vcore_context());
	assert(uthread->state == UT_RUNNING);
	/* Pass info to ourselves across the uth_yield -> __uth_yield transition. */
	uthread->yield_func = yield_func;
	uthread->yield_arg = yield_arg;
	/* Don't migrate this thread to another vcore, since it depends on being on
	 * the same vcore throughout (once it disables notifs).  The race is that we
	 * read vcoreid, then get interrupted / migrated before disabling notifs. */
	uthread->flags |= UTHREAD_DONT_MIGRATE;
	cmb();	/* don't let DONT_MIGRATE write pass the vcoreid read */
	uint32_t vcoreid = vcore_id();
	printd("[U] Uthread %08p is yielding on vcore %d\n", uthread, vcoreid);
	struct preempt_data *vcpd = vcpd_of(vcoreid);
	/* once we do this, we might miss a notif_pending, so we need to enter vcore
	 * entry later.  Need to disable notifs so we don't get in weird loops with
	 * save_user_ctx() and pop_user_ctx(). */
	disable_notifs(vcoreid);
	/* take the current state and save it into t->utf when this pthread
	 * restarts, it will continue from right after this, see yielding is false,
	 * and short ciruit the function.  Don't do this if we're dying. */
	if (save_state) {
		/* Need to signal this before we actually save, since save_user_ctx
		 * returns() twice (once now, once when woken up) */
		uthread->flags |= UTHREAD_SAVED;
		save_user_ctx(&uthread->u_ctx);
	}
	cmb();	/* Force reread of yielding. Technically save_user_ctx() suffices*/
	/* Restart path doesn't matter if we're dying */
	if (!yielding)
		goto yield_return_path;
	/* From here on down is only executed on the save path (not the wake up) */
	yielding = FALSE; /* for when it starts back up */
	/* TODO: remove this when all arches support SW contexts */
	if (save_state && (uthread->u_ctx.type != ROS_SW_CTX)) {
		save_fp_state(&uthread->as);
		uthread->flags |= UTHREAD_FPSAVED;
	}
	/* Change to the transition context (both TLS (if applicable) and stack). */
	if (__uthread_has_tls(uthread)) {
		set_tls_desc(get_vcpd_tls_desc(vcoreid), vcoreid);
		assert(current_uthread == uthread);
		assert(in_vcore_context());
	} else {
		/* Since uthreads and vcores share TLS (it's always the vcore's TLS, the
		 * uthread one just bootstraps from it), we need to change our state at
		 * boundaries between the two 'contexts' */
		__vcore_context = TRUE;
	}
	/* After this, make sure you don't use local variables.  Also, make sure the
	 * compiler doesn't use them without telling you (TODO).
	 *
	 * In each arch's set_stack_pointer, make sure you subtract off as much room
	 * as you need to any local vars that might be pushed before calling the
	 * next function, or for whatever other reason the compiler/hardware might
	 * walk up the stack a bit when calling a noreturn function. */
	set_stack_pointer((void*)vcpd->transition_stack);
	/* Finish exiting in another function. */
	__uthread_yield();
	/* Should never get here */
	assert(0);
	/* Will jump here when the uthread's trapframe is restarted/popped. */
yield_return_path:
	printd("[U] Uthread %08p returning from a yield!\n", uthread);
}

/* We explicitly don't support sleep(), since old callers of it have
 * expectations of being woken up by signal handlers.  If we need that, we can
 * build it in to sleep() later.  If you just want to sleep for a while, call
 * this helper. */
void uthread_sleep(unsigned int seconds)
{
	sys_block(seconds * 1000000);	/* usec sleep */
}

/* Cleans up the uthread (the stuff we did in uthread_init()).  If you want to
 * destroy a currently running uthread, you'll want something like
 * pthread_exit(), which yields, and calls this from its sched_ops yield. */
void uthread_cleanup(struct uthread *uthread)
{
	printd("[U] thread %08p on vcore %d is DYING!\n", uthread, vcore_id());
	/* we alloc and manage the TLS, so lets get rid of it, except for thread0.
	 * glibc owns it.  might need to keep it around for a full exit() */
	if (__uthread_has_tls(uthread) && !(uthread->flags & UTHREAD_IS_THREAD0))
		__uthread_free_tls(uthread);
}

static void __ros_syscall_spinon(struct syscall *sysc)
{
	while (!(atomic_read(&sysc->flags) & (SC_DONE | SC_PROGRESS)))
		cpu_relax();
}

/* Attempts to block on sysc, returning when it is done or progress has been
 * made. */
void __ros_mcp_syscall_blockon(struct syscall *sysc)
{
	/* even if we are in 'vcore context', an _S can block */
	if (!in_multi_mode()) {
		__ros_scp_syscall_blockon(sysc);
		return;
	}
	/* MCP vcore's don't know what to do yet, so we have to spin */
	if (in_vcore_context()) {
		__ros_syscall_spinon(sysc);
		return;
	}
	/* At this point, we know we're a uthread in an MCP.  If we're a
	 * DONT_MIGRATE uthread, then it's disabled notifs and is basically in
	 * vcore context, enough so that it can't call into the 2LS. */
	assert(current_uthread);
	if (current_uthread->flags & UTHREAD_DONT_MIGRATE) {
		assert(!notif_is_enabled(vcore_id()));	/* catch bugs */
		__ros_syscall_spinon(sysc);
	}
	/* double check before doing all this crap */
	if (atomic_read(&sysc->flags) & (SC_DONE | SC_PROGRESS))
		return;
	/* for both debugging and syscall cancelling */
	current_uthread->sysc = sysc;
	/* yield, calling 2ls-blockon(cur_uth, sysc) on the other side */
	uthread_yield(TRUE, sched_ops->thread_blockon_sysc, sysc);
}

/* Simply sets current uthread to be whatever the value of uthread is.  This
 * can be called from outside of sched_entry() to highjack the current context,
 * and make sure that the new uthread struct is used to store this context upon
 * yielding, etc. USE WITH EXTREME CAUTION! */
void highjack_current_uthread(struct uthread *uthread)
{
	uint32_t vcoreid = vcore_id();
	assert(uthread != current_uthread);
	current_uthread->state = UT_NOT_RUNNING;
	uthread->state = UT_RUNNING;
	/* Make sure the vcore is tracking the new uthread struct */
	if (__uthread_has_tls(current_uthread))
		vcore_set_tls_var(current_uthread, uthread);
	else
		current_uthread = uthread;
	/* and make sure we are using the correct TLS for the new uthread */
	if (__uthread_has_tls(uthread)) {
		assert(uthread->tls_desc);
		set_tls_desc(uthread->tls_desc, vcoreid);
		__vcoreid = vcoreid;	/* setting the uthread's TLS var */
	}
}

/* Helper: loads a uthread's TLS on this vcore, if applicable.  If our uthreads
 * do not have their own TLS, we simply switch the __vc_ctx, signalling that the
 * context running here is (soon to be) a uthread. */
static void set_uthread_tls(struct uthread *uthread, uint32_t vcoreid)
{
	if (__uthread_has_tls(uthread)) {
		set_tls_desc(uthread->tls_desc, vcoreid);
		__vcoreid = vcoreid;	/* setting the uthread's TLS var */
	} else {
		__vcore_context = FALSE;
	}
}

/* Run the thread that was current_uthread, from a previous run.  Should be
 * called only when the uthread already was running, and we were interrupted by
 * the kernel (event, etc).  Do not call this to run a fresh uthread, even if
 * you've set it to be current. */
void run_current_uthread(void)
{
	uint32_t vcoreid = vcore_id();
	struct preempt_data *vcpd = vcpd_of(vcoreid);
	assert(current_uthread);
	assert(current_uthread->state == UT_RUNNING);
	/* Uth was already running, should not have been saved */
	assert(!(current_uthread->flags & UTHREAD_SAVED));
	assert(!(current_uthread->flags & UTHREAD_FPSAVED));
	printd("[U] Vcore %d is restarting uthread %08p\n", vcoreid,
	       current_uthread);
	/* Go ahead and start the uthread */
	set_uthread_tls(current_uthread, vcoreid);
	/* Run, using the TF in the VCPD.  FP state should already be loaded */
	pop_user_ctx(&vcpd->uthread_ctx, vcoreid);
	assert(0);
}

/* Launches the uthread on the vcore.  Don't call this on current_uthread. 
 *
 * In previous versions of this, we used to check for events after setting
 * current_uthread.  That is super-dangerous.  handle_events() doesn't always
 * return (which we used to handle), and it may also clear current_uthread.  We
 * needed to save uthread in current_uthread, in case we didn't return.  If we
 * didn't return, the vcore started over at vcore_entry, with current set.  When
 * this happens, we never actually had loaded cur_uth's FP and GP onto the core,
 * so cur_uth fails.  Check out 4602599be for more info.
 *
 * Ultimately, handling events again in these 'popping helpers' isn't even
 * necessary (we only must do it once for an entire time in VC ctx, and in
 * loops), and might have been optimizing a rare event at a cost in both
 * instructions and complexity. */
void run_uthread(struct uthread *uthread)
{
	uint32_t vcoreid = vcore_id();
	struct preempt_data *vcpd = vcpd_of(vcoreid);
	assert(!current_uthread);
	assert(uthread->state == UT_NOT_RUNNING);
	assert(uthread->flags & UTHREAD_SAVED);
	/* For HW CTX, FPSAVED must match UTH SAVE (and both be on here).  For SW,
	 * FP should never be saved. */
	if (uthread->u_ctx.type == ROS_HW_CTX)
		assert(uthread->flags & UTHREAD_FPSAVED);
	else
		assert(!(uthread->flags & UTHREAD_FPSAVED));
	uthread->state = UT_RUNNING;
	/* Save a ptr to the uthread we'll run in the transition context's TLS */
	current_uthread = uthread;
	if (uthread->flags & UTHREAD_FPSAVED) {
		uthread->flags &= ~UTHREAD_FPSAVED;
		restore_fp_state(&uthread->as);
	}
	set_uthread_tls(uthread, vcoreid);
	/* the uth's context will soon be in the cpu (or VCPD), no longer saved */
	uthread->flags &= ~UTHREAD_SAVED;
	pop_user_ctx(&uthread->u_ctx, vcoreid);
	assert(0);
}

/* Runs the uthread, but doesn't care about notif pending.  Only call this when
 * there was a DONT_MIGRATE uthread, or a similar situation where the uthread
 * will check messages soon (like calling enable_notifs()). */
static void __run_current_uthread_raw(void)
{
	uint32_t vcoreid = vcore_id();
	struct preempt_data *vcpd = vcpd_of(vcoreid);
	/* We need to manually say we have a notif pending, so we eventually return
	 * to vcore context.  (note the kernel turned it off for us) */
	vcpd->notif_pending = TRUE;
	assert(!(current_uthread->flags & UTHREAD_SAVED));
	assert(!(current_uthread->flags & UTHREAD_FPSAVED));
	set_uthread_tls(current_uthread, vcoreid);
	pop_user_ctx_raw(&vcpd->uthread_ctx, vcoreid);
	assert(0);
}

/* Copies the uthread trapframe and silly state from the vcpd to the uthread,
 * subject to the uthread's flags and whatnot.
 *
 * For example: The uthread state might still be in the uthread struct.  Imagine
 * the 2LS decides to run a new uthread and sets it up as current, but doesn't
 * actually run it yet.  The 2LS happened to voluntarily give up the VC (due to
 * some other event) and then wanted to copy out the thread.  This is pretty
 * rare - the normal case is when an IRQ of some sort hit the core and the
 * kernel copied the running state into VCPD.
 *
 * The FP state could also be in VCPD (e.g. preemption being handled remotely),
 * it could be in the uthread struct (e.g. hasn't started running yet) or even
 * in the FPU (e.g. took an IRQ/notif and we're handling the preemption of
 * another vcore).
 *
 * There are some cases where we'll have a uthread SW ctx that needs to be
 * copied out: uth syscalls, notif happens, and the core comes back from the
 * kernel in VC ctx.  VC ctx calls copy_out (response to preempt_pending or done
 * while handling a preemption). */
static void copyout_uthread(struct preempt_data *vcpd, struct uthread *uthread,
                            bool vcore_local)
{
	assert(uthread);
	if (uthread->flags & UTHREAD_SAVED) {
		/* I don't know of scenarios where HW ctxs FP state differs from GP */
		if (uthread->u_ctx.type == ROS_HW_CTX)
			assert(uthread->flags & UTHREAD_FPSAVED);
		assert(vcore_local);
		return;
	}
	/* If we're copying GP state, it must be in VCPD */
	uthread->u_ctx = vcpd->uthread_ctx;
	uthread->flags |= UTHREAD_SAVED;
	printd("VC %d copying out uthread %08p\n", vcore_id(), uthread);
	/* Software contexts do not need FP state, nor should we think it has any */
	if (uthread->u_ctx.type == ROS_SW_CTX) {
		assert(!(uthread->flags & UTHREAD_FPSAVED));
		return;
	}
	/* HW contexts also should not have it saved either.  Should be either in
	 * the VCPD or the FPU.  Yes, this is the same assert. */
	assert(!(uthread->flags & UTHREAD_FPSAVED));
	/* When we're dealing with the uthread running on our own vcore, the FP
	 * state is in the actual FPU, not VCPD.  It might also be in VCPD, but it
	 * will always be in the FPU (the kernel maintains this for us, in the event
	 * we were preempted since the uthread was last running). */
	if (vcore_local)
		save_fp_state(&uthread->as);
	else
		uthread->as = vcpd->preempt_anc;
	uthread->flags |= UTHREAD_FPSAVED;
}

/* Helper, packages up and pauses a uthread that was running on vcoreid.  Used
 * by preemption handling (and detection) so far.  Careful using this, esp if
 * it is on another vcore (need to make sure it's not running!).  If you are
 * using it on the local vcore, set vcore_local = TRUE. */
static void __uthread_pause(struct preempt_data *vcpd, struct uthread *uthread,
                            bool vcore_local)
{
	assert(!(uthread->flags & UTHREAD_DONT_MIGRATE));
	copyout_uthread(vcpd, uthread, vcore_local);
	uthread->state = UT_NOT_RUNNING;
	/* Call out to the 2LS to package up its uthread */
	assert(sched_ops->thread_paused);
	sched_ops->thread_paused(uthread);
}

/* Deals with a pending preemption (checks, responds).  If the 2LS registered a
 * function, it will get run.  Returns true if you got preempted.  Called
 * 'check' instead of 'handle', since this isn't an event handler.  It's the "Oh
 * shit a preempt is on its way ASAP".
 *
 * Be careful calling this: you might not return, so don't call it if you can't
 * handle that.  If you are calling this from an event handler, you'll need to
 * do things like ev_might_not_return().  If the event can via an INDIR ev_q,
 * that ev_q must be a NOTHROTTLE.
 *
 * Finally, don't call this from a place that might have a DONT_MIGRATE
 * cur_uth.  This should be safe for most 2LS code. */
bool __check_preempt_pending(uint32_t vcoreid)
{
	bool retval = FALSE;
	assert(in_vcore_context());
	if (__preempt_is_pending(vcoreid)) {
		retval = TRUE;
		if (sched_ops->preempt_pending)
			sched_ops->preempt_pending();
		/* If we still have a cur_uth, copy it out and hand it back to the 2LS
		 * before yielding. */
		if (current_uthread) {
			__uthread_pause(vcpd_of(vcoreid), current_uthread, TRUE);
			current_uthread = 0;
		}
		/* vcore_yield tries to yield, and will pop back up if this was a spurious
		 * preempt_pending or if it handled an event.  For now, we'll just keep
		 * trying to yield so long as a preempt is coming in.  Eventually, we'll
		 * handle all of our events and yield, or else the preemption will hit
		 * and someone will recover us (at which point we'll break out of the
		 * loop) */
		while (__procinfo.vcoremap[vcoreid].preempt_pending) {
			vcore_yield(TRUE);
			cpu_relax();
		}
	}
	return retval;
}

/* Helper: This is a safe way for code to disable notifs if it *might* be called
 * from uthread context (like from a notif_safe lock).  Pair this with
 * uth_enable_notifs() unless you know what you're doing. */
void uth_disable_notifs(void)
{
	if (!in_vcore_context() && in_multi_mode()) {
		if (current_uthread)
			current_uthread->flags |= UTHREAD_DONT_MIGRATE;
		cmb();	/* don't issue the flag write before the vcore_id() read */
		disable_notifs(vcore_id());
	}
}

/* Helper: Pair this with uth_disable_notifs(). */
void uth_enable_notifs(void)
{
	if (!in_vcore_context() && in_multi_mode()) {
		if (current_uthread)
			current_uthread->flags &= ~UTHREAD_DONT_MIGRATE;
		cmb();	/* don't enable before ~DONT_MIGRATE */
		enable_notifs(vcore_id());
	}
}

/* Helper: returns TRUE if it succeeded in starting the uth stealing process. */
static bool start_uth_stealing(struct preempt_data *vcpd)
{
	long old_flags;
	do {
		old_flags = atomic_read(&vcpd->flags);
		/* Spin if the kernel is mucking with the flags */
		while (old_flags & VC_K_LOCK)
			old_flags = atomic_read(&vcpd->flags);
		/* Someone else is stealing, we failed */
		if (old_flags & VC_UTHREAD_STEALING)
			return FALSE;
	} while (!atomic_cas(&vcpd->flags, old_flags,
	                     old_flags | VC_UTHREAD_STEALING));
	return TRUE;
}

/* Helper: pairs with stop_uth_stealing */
static void stop_uth_stealing(struct preempt_data *vcpd)
{
	long old_flags;
	do {
		old_flags = atomic_read(&vcpd->flags);
		assert(old_flags & VC_UTHREAD_STEALING);	/* sanity */
		while (old_flags & VC_K_LOCK)
			old_flags = atomic_read(&vcpd->flags);
	} while (!atomic_cas(&vcpd->flags, old_flags,
	                     old_flags & ~VC_UTHREAD_STEALING));
}

/* Handles INDIRS for another core (the public mbox).  We synchronize with the
 * kernel (__set_curtf_to_vcoreid). */
static void handle_indirs(uint32_t rem_vcoreid)
{
	long old_flags;
	struct preempt_data *rem_vcpd = vcpd_of(rem_vcoreid);
	/* Turn off their message reception if they are still preempted.  If they
	 * are no longer preempted, we do nothing - they will handle their own
	 * messages.  Turning on CAN_RCV will route this vcore's messages to
	 * fallback vcores (if applicable). */
	do {
		old_flags = atomic_read(&rem_vcpd->flags);
		while (old_flags & VC_K_LOCK)
			old_flags = atomic_read(&rem_vcpd->flags);
		if (!(old_flags & VC_PREEMPTED))
			return;
	} while (!atomic_cas(&rem_vcpd->flags, old_flags,
	                     old_flags & ~VC_CAN_RCV_MSG));
	wrmb();	/* don't let the CAN_RCV write pass reads of the mbox status */
	/* handle all INDIRs of the remote vcore */
	handle_vcpd_mbox(rem_vcoreid);
}

/* Helper.  Will ensure a good attempt at changing vcores, meaning we try again
 * if we failed for some reason other than the vcore was already running. */
static void __change_vcore(uint32_t rem_vcoreid, bool enable_my_notif)
{
	/* okay to do a normal spin/relax here, even though we are in vcore
	 * context. */
	while (-EAGAIN == sys_change_vcore(rem_vcoreid, enable_my_notif))
		cpu_relax();
}

/* Helper, used in preemption recovery.  When you can freely leave vcore
 * context and need to change to another vcore, call this.  vcpd is the caller,
 * rem_vcoreid is the remote vcore.  This will try to package up your uthread.
 * It may return, either because the other core already started up (someone else
 * got it), or in some very rare cases where we had to stay in our vcore
 * context */
static void change_to_vcore(struct preempt_data *vcpd, uint32_t rem_vcoreid)
{
	bool were_handling_remotes;
	/* Unlikely, but if we have no uthread we can just change.  This is the
	 * check, sync, then really check pattern: we can only really be sure about
	 * current_uthread after we check STEALING. */
	if (!current_uthread) {
		/* there might be an issue with doing this while someone is recovering.
		 * once they 0'd it, we should be good to yield.  just a bit dangerous.
		 * */
		were_handling_remotes = ev_might_not_return();
		__change_vcore(rem_vcoreid, TRUE);	/* noreturn on success */
		goto out_we_returned;
	}
	/* Note that the reason we need to check STEALING is because we can get into
	 * vcore context and slip past that check in vcore_entry when we are
	 * handling a preemption message.  Anytime preemption recovery cares about
	 * the calling vcore's cur_uth, it needs to be careful about STEALING.  But
	 * it is safe to do the check up above (if it's 0, it won't concurrently
	 * become non-zero).
	 *
	 * STEALING might be turned on at any time.  Whoever turns it on will do
	 * nothing if we are online or were in vc_ctx.  So if it is on, we can't
	 * touch current_uthread til it is turned off (not sure what state they saw
	 * us in).  We could spin here til they unset STEALING (since they will
	 * soon), but there is a chance they were preempted, so we need to make
	 * progress by doing a sys_change_vcore(). */
	/* Crap, someone is stealing (unlikely).  All we can do is change. */
	if (atomic_read(&vcpd->flags) & VC_UTHREAD_STEALING) {
		__change_vcore(rem_vcoreid, FALSE);	/* returns on success */
		return;
	}
	cmb();
	/* Need to recheck, in case someone stole it and finished before we checked
	 * VC_UTHREAD_STEALING. */
	if (!current_uthread) {
		were_handling_remotes = ev_might_not_return();
		__change_vcore(rem_vcoreid, TRUE);	/* noreturn on success */
		goto out_we_returned;
	}
	/* Need to make sure we don't have a DONT_MIGRATE (very rare, someone would
	 * have to steal from us to get us to handle a preempt message, and then had
	 * to finish stealing (and fail) fast enough for us to miss the previous
	 * check). */
	if (current_uthread->flags & UTHREAD_DONT_MIGRATE) {
		__change_vcore(rem_vcoreid, FALSE);	/* returns on success */
		return;
	}
	/* Now save our uthread and restart them */
	assert(current_uthread);
	__uthread_pause(vcpd, current_uthread, TRUE);
	current_uthread = 0;
	were_handling_remotes = ev_might_not_return();
	__change_vcore(rem_vcoreid, TRUE);		/* noreturn on success */
	/* Fall-through to out_we_returned */
out_we_returned:
	ev_we_returned(were_handling_remotes);
}

/* This handles a preemption message.  When this is done, either we recovered,
 * or recovery *for our message* isn't needed. */
void handle_vc_preempt(struct event_msg *ev_msg, unsigned int ev_type)
{
	uint32_t vcoreid = vcore_id();
	struct preempt_data *vcpd = vcpd_of(vcoreid);
	uint32_t rem_vcoreid = ev_msg->ev_arg2;
	struct preempt_data *rem_vcpd = vcpd_of(rem_vcoreid);
	struct uthread *uthread_to_steal = 0;
	struct uthread **rem_cur_uth;
	bool cant_migrate = FALSE;

	assert(in_vcore_context());
	/* Just drop messages about ourselves.  They are old.  If we happen to be
	 * getting preempted right now, there's another message out there about
	 * that. */
	if (rem_vcoreid == vcoreid)
		return;
	printd("Vcore %d was preempted (i'm %d), it's flags %08p!\n",
	       ev_msg->ev_arg2, vcoreid, rem_vcpd->flags);
	/* Spin til the kernel is done with flags.  This is how we avoid handling
	 * the preempt message before the preemption. */
	while (atomic_read(&rem_vcpd->flags) & VC_K_LOCK)
		cpu_relax();
	/* If they aren't preempted anymore, just return (optimization). */
	if (!(atomic_read(&rem_vcpd->flags) & VC_PREEMPTED))
		return;
	/* At this point, we need to try to recover */
	/* This case handles when the remote core was in vcore context */
	if (rem_vcpd->notif_disabled) {
		printd("VC %d recovering %d, notifs were disabled\n", vcoreid,
		       rem_vcoreid);
		change_to_vcore(vcpd, rem_vcoreid);
		return;	/* in case it returns.  we've done our job recovering */
	}
	/* So now it looks like they were not in vcore context.  We want to steal
	 * the uthread.  Set stealing, then doublecheck everything.  If stealing
	 * fails, someone else is stealing and we can just leave.  That other vcore
	 * who is stealing will check the VCPD/INDIRs when it is done. */
	if (!start_uth_stealing(rem_vcpd))
		return;
	/* Now we're stealing.  Double check everything.  A change in preempt status
	 * or notif_disable status means the vcore has since restarted.  The vcore
	 * may or may not have started after we set STEALING.  If it didn't, we'll
	 * need to bail out (but still check messages, since above we assumed the
	 * uthread stealer handles the VCPD/INDIRs).  Since the vcore is running, we
	 * don't need to worry about handling the message any further.  Future
	 * preemptions will generate another message, so we can ignore getting the
	 * uthread or anything like that. */
	printd("VC %d recovering %d, trying to steal uthread\n", vcoreid,
	       rem_vcoreid);
	if (!(atomic_read(&rem_vcpd->flags) & VC_PREEMPTED))
		goto out_stealing;
	/* Might be preempted twice quickly, and the second time had notifs
	 * disabled.
	 *
	 * Also note that the second preemption event had another
	 * message sent, which either we or someone else will deal with.  And also,
	 * we don't need to worry about how we are stealing still and plan to
	 * abort.  If another vcore handles that second preemption message, either
	 * the original vcore is in vc ctx or not.  If so, we bail out and the
	 * second preemption handling needs to change_to.  If not, we aren't
	 * bailing out, and we'll handle the preemption as normal, and the second
	 * handler will bail when it fails to steal. */
	if (rem_vcpd->notif_disabled)
		goto out_stealing;
	/* At this point, we're clear to try and steal the uthread.  We used to
	 * switch to their TLS to steal the uthread, but we can access their
	 * current_uthread directly. */
	rem_cur_uth = get_cur_uth_addr(rem_vcoreid);
	uthread_to_steal = *rem_cur_uth;
	if (uthread_to_steal) {
		/* Extremely rare: they have a uthread, but it can't migrate.  So we'll
		 * need to change to them. */
		if (uthread_to_steal->flags & UTHREAD_DONT_MIGRATE) {
			printd("VC %d recovering %d, can't migrate uthread!\n", vcoreid,
			       rem_vcoreid);
			stop_uth_stealing(rem_vcpd);
			change_to_vcore(vcpd, rem_vcoreid);
			return;	/* in case it returns.  we've done our job recovering */
		} else {
			*rem_cur_uth = 0;
			/* we're clear to steal it */
			printd("VC %d recovering %d, uthread %08p stolen\n", vcoreid,
			       rem_vcoreid, uthread_to_steal);
			__uthread_pause(rem_vcpd, uthread_to_steal, FALSE);
			/* can't let the cur_uth = 0 write and any writes from __uth_pause()
			 * to pass stop_uth_stealing. */
			wmb();
		}
	}
	/* Fallthrough */
out_stealing:
	stop_uth_stealing(rem_vcpd);
	handle_indirs(rem_vcoreid);
}

/* This handles a "check indirs" message.  When this is done, either we checked
 * their indirs, or the vcore restarted enough so that checking them is
 * unnecessary.  If that happens and they got preempted quickly, then another
 * preempt/check_indirs was sent out. */
void handle_vc_indir(struct event_msg *ev_msg, unsigned int ev_type)
{
	uint32_t vcoreid = vcore_id();
	uint32_t rem_vcoreid = ev_msg->ev_arg2;

	if (rem_vcoreid == vcoreid)
		return;
	handle_indirs(rem_vcoreid);
}

/* Attempts to register ev_q with sysc, so long as sysc is not done/progress.
 * Returns true if it succeeded, and false otherwise.  False means that the
 * syscall is done, and does not need an event set (and should be handled
 * accordingly).
 * 
 * A copy of this is in glibc/sysdeps/ros/syscall.c.  Keep them in sync. */
bool register_evq(struct syscall *sysc, struct event_queue *ev_q)
{
	int old_flags;
	sysc->ev_q = ev_q;
	wrmb();	/* don't let that write pass any future reads (flags) */
	/* Try and set the SC_UEVENT flag (so the kernel knows to look at ev_q) */
	do {
		/* no cmb() needed, the atomic_read will reread flags */
		old_flags = atomic_read(&sysc->flags);
		/* Spin if the kernel is mucking with syscall flags */
		while (old_flags & SC_K_LOCK)
			old_flags = atomic_read(&sysc->flags);
		/* If the kernel finishes while we are trying to sign up for an event,
		 * we need to bail out */
		if (old_flags & (SC_DONE | SC_PROGRESS)) {
			sysc->ev_q = 0;		/* not necessary, but might help with bugs */
			return FALSE;
		}
	} while (!atomic_cas(&sysc->flags, old_flags, old_flags | SC_UEVENT));
	return TRUE;
}

/* De-registers a syscall, so that the kernel will not send an event when it is
 * done.  The call could already be SC_DONE, or could even finish while we try
 * to unset SC_UEVENT.
 *
 * There is a chance the kernel sent an event if you didn't do this in time, but
 * once this returns, the kernel won't send a message.
 *
 * If the kernel is trying to send a message right now, this will spin (on
 * SC_K_LOCK).  We need to make sure we deregistered, and that if a message
 * is coming, that it already was sent (and possibly overflowed), before
 * returning. */
void deregister_evq(struct syscall *sysc)
{
	int old_flags;
	sysc->ev_q = 0;
	wrmb();	/* don't let that write pass any future reads (flags) */
	/* Try and unset the SC_UEVENT flag */
	do {
		/* no cmb() needed, the atomic_read will reread flags */
		old_flags = atomic_read(&sysc->flags);
		/* Spin if the kernel is mucking with syscall flags */
		while (old_flags & SC_K_LOCK)
			old_flags = atomic_read(&sysc->flags);
		/* Note we don't care if the SC_DONE flag is getting set.  We just need
		 * to avoid clobbering flags */
	} while (!atomic_cas(&sysc->flags, old_flags, old_flags & ~SC_UEVENT));
}

static inline bool __uthread_has_tls(struct uthread *uthread)
{
	return uthread->tls_desc != UTH_TLSDESC_NOTLS;
}

/* TLS helpers */
static int __uthread_allocate_tls(struct uthread *uthread)
{
	assert(!uthread->tls_desc);
	uthread->tls_desc = allocate_tls();
	if (!uthread->tls_desc) {
		errno = ENOMEM;
		return -1;
	}
	return 0;
}

static int __uthread_reinit_tls(struct uthread *uthread)
{
	uthread->tls_desc = reinit_tls(uthread->tls_desc);
	if (!uthread->tls_desc) {
		errno = ENOMEM;
		return -1;
	}
	return 0;
}

static void __uthread_free_tls(struct uthread *uthread)
{
	free_tls(uthread->tls_desc);
	uthread->tls_desc = NULL;
}