[Qemu-devel] [PATCH] replay: wake up vCPU when replaying

[Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>]

In record/replay icount mode vCPU thread and iothread synchronize
the execution using the checkpoints.
vCPU thread processes the virtual timers and iothread processes all others.
When iothread wants to wake up sleeping vCPU thread, it sends dummy queued
work. Therefore it could be the following sequence of the events in
record mode:
 - IO: sending dummy work
 - IO: processing timers
 - CPU: wakeup
 - CPU: clearing dummy work
 - CPU: processing virtual timers

But due to the races in replay mode the sequence may change:
 - IO: sending dummy work
 - CPU: wakeup
 - CPU: clearing dummy work
 - CPU: sleeping again because nothing to do
 - IO: Processing timers
 - CPU: zzzz

In this case vCPU will not wake up, because dummy work is not to be set up
again.

This patch tries to wake up the vCPU when it sleeps and the icount warp
checkpoint isn't met. It means that vCPU has something to do, because
there are no other reasons of non-matching warp checkpoint.

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>
---
 cpus.c |   15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/cpus.c b/cpus.c
index 181ce33..bad6a33 100644
--- a/cpus.c
+++ b/cpus.c
@@ -539,11 +539,6 @@ void qemu_start_warp_timer(void)
         return;
     }
 
-    /* warp clock deterministically in record/replay mode */
-    if (!replay_checkpoint(CHECKPOINT_CLOCK_WARP_START)) {
-        return;
-    }
-
     if (!all_cpu_threads_idle()) {
         return;
     }
@@ -553,6 +548,16 @@ void qemu_start_warp_timer(void)
         return;
     }
 
+    /* warp clock deterministically in record/replay mode */
+    if (!replay_checkpoint(CHECKPOINT_CLOCK_WARP_START)) {
+        /* vCPU is sleeping and warp can't be started.
+           It is probably a race condition: notification sent
+           to vCPU was processed in advance and vCPU went to sleep.
+           Therefore we have to wake it up for doing someting. */
+        qemu_clock_notify(QEMU_CLOCK_VIRTUAL);
+        return;
+    }
+
     /* We want to use the earliest deadline from ALL vm_clocks */
     clock = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL_RT);
     deadline = qemu_clock_deadline_ns_all(QEMU_CLOCK_VIRTUAL);