[Devel] [PATCH rh7 v1 27/36] ms/kvm/x86: Remove Hyper-V SynIC timer stopping

[Andrey Smetanin]

It's possible that guest send us Hyper-V EOM at the middle
of Hyper-V SynIC timer running, so we start processing of Hyper-V
SynIC timers in vcpu context and stop the Hyper-V SynIC timer
unconditionally:

    host                                       guest
    ------------------------------------------------------------------------------
                                           start periodic stimer
    start periodic timer
    timer expires after 15ms
    send expiration message into guest
    restart periodic timer
    timer expires again after 15 ms
    msg slot is still not cleared so
    setup ->msg_pending
(1) restart periodic timer
                                           process timer msg and clear slot
                                           ->msg_pending was set:
                                               send EOM into host
    received EOM
      kvm_make_request(KVM_REQ_HV_STIMER)

    kvm_hv_process_stimers():
        ...
        stimer_stop()
        if (time_now >= stimer->exp_time)
                stimer_expiration(stimer);

Because the timer was rearmed at (1), time_now < stimer->exp_time
and stimer_expiration is not called.  The timer then never fires.

The patch fixes such situation by not stopping Hyper-V SynIC timer
at all, because it's safe to restart it without stop in vcpu context
and timer callback always returns HRTIMER_NORESTART.

ms commit - 481d2bcc8454a44811db2bb68ac216fc6c5a23db.

Signed-off-by: Andrey Smetanin <asmetanin at virtuozzo.com>
CC: Gleb Natapov <gleb at kernel.org>
CC: Paolo Bonzini <pbonzini at redhat.com>
CC: Roman Kagan <rkagan at virtuozzo.com>
CC: Denis V. Lunev <den at openvz.org>
CC: qemu-devel at nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
---
 arch/x86/kvm/hyperv.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 8ff8829..f34f666 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -598,7 +598,6 @@ void kvm_hv_process_stimers(struct kvm_vcpu *vcpu)
 	for (i = 0; i < ARRAY_SIZE(hv_vcpu->stimer); i++)
 		if (test_and_clear_bit(i, hv_vcpu->stimer_pending_bitmap)) {
 			stimer = &hv_vcpu->stimer[i];
-			stimer_stop(stimer);
 			if (stimer->config & HV_STIMER_ENABLE) {
 				time_now = get_time_ref_counter(vcpu->kvm);
 				if (time_now >= stimer->exp_time)
-- 
2.4.3