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[Qemu-devel] [PATCH] vl: Move cpu_synchronize_all_states() into qemu_sys

From: David Gibson
Subject: [Qemu-devel] [PATCH] vl: Move cpu_synchronize_all_states() into qemu_system_reset()
Date: Mon, 4 Apr 2016 14:49:11 +1000

There are currently 3 calls to qemu_system_reset() in vl.c.  Two of them
are immediately preceded by a cpu_synchronize_all_states9) and the
remaining one should be.

The one which doesn't is the very first reset called directly from main().
Without a cpu_synchronize_all_states(), kvm_vcpu_dirty is false at this
point from the earlier cpu_synchronize_all_post_init().  That's incorrect
because the reset path is quite likely to update the CPU state, and that
updated state should be pushed back to KVM, not overwritten with stale
data pushed to KVM immediately after init.

This patch moves the call to cpu_synchronize_all_states() into
qemu_system_reset() for safety, so it is always called.  AFAICT this should
be safe for the handful of callers outside vl.c - these all appear to be in
places where the cpu state is already synchronized so the extra call
will be a no-op.

Signed-off-by: David Gibson <address@hidden>
 vl.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

This fixes a real but on ppc - the incorrect state of kvm_vcpu_dirty
means that an extra cpu_synchronize_state() in revised code for
updating the MSR is not a no-op as expected and loads a stale MSR
value, resulting in an incorrect MSR value on entry to the guest.

Therefore, I'm hoping to apply this ASAP to 2.6.

Laurent, could you please verify that this does indeed address the
problem with an incorrect MSR on entry.

diff --git a/vl.c b/vl.c
index bd81ea9..3629336 100644
--- a/vl.c
+++ b/vl.c
@@ -1745,6 +1745,8 @@ void qemu_system_reset(bool report)
     mc = current_machine ? MACHINE_GET_CLASS(current_machine) : NULL;
+    cpu_synchronize_all_states();
     if (mc && mc->reset) {
     } else {
@@ -1893,7 +1895,6 @@ static bool main_loop_should_exit(void)
     if (qemu_reset_requested()) {
-        cpu_synchronize_all_states();
         if (!runstate_check(RUN_STATE_RUNNING) &&
@@ -1903,7 +1904,6 @@ static bool main_loop_should_exit(void)
     if (qemu_wakeup_requested()) {
-        cpu_synchronize_all_states();
         notifier_list_notify(&wakeup_notifiers, &wakeup_reason);
         wakeup_reason = QEMU_WAKEUP_REASON_NONE;

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