[PATCH] x86/kvm/hyper-v: avoid spurious pending stimer on vCPU init

[Vitaly Kuznetsov <vkuznets@redhat.com>]

When userspace initializes guest vCPUs it may want to zero all supported
MSRs including Hyper-V related ones including HV_X64_MSR_STIMERn_CONFIG/
HV_X64_MSR_STIMERn_COUNT. With commit f3b138c5d89a ("kvm/x86: Update SynIC
timers on guest entry only") we began doing stimer_mark_pending()
unconditionally on every config change.

The issue I'm observing manifests itself as following:
- Qemu writes 0 to STIMERn_{CONFIG,COUNT} MSRs and marks all stimers as
  pending in stimer_pending_bitmap, arms KVM_REQ_HV_STIMER;
- kvm_hv_has_stimer_pending() starts returning true;
- kvm_vcpu_has_events() starts returning true;
- kvm_arch_vcpu_runnable() starts returning true;
- when kvm_arch_vcpu_ioctl_run() gets into
  (vcpu->arch.mp_state == KVM_MP_STATE_UNINITIALIZED) case:
  - kvm_vcpu_block() gets in 'kvm_vcpu_check_block(vcpu) < 0' and returns
    immediately, avoiding normal wait path;
  - -EAGAIN is returned from kvm_arch_vcpu_ioctl_run() immediately forcing
    userspace to retry.

So instead of normal wait path we get a busy loop on all secondary vCPUs
before they get INIT signal. This seems to be undesirable, especially given
that this happens even when Hyper-V extensions are not used.

Generally, it seems to be pointless to mark an stimer as pending in
stimer_pending_bitmap and arm KVM_REQ_HV_STIMER as the only thing
kvm_hv_process_stimers() will do is clear the corresponding bit. We may
just not mark disabled timers as pending instead.

Fixes: f3b138c5d89a ("kvm/x86: Update SynIC timers on guest entry only")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 arch/x86/kvm/hyperv.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 89d20ed1d2e8..371c669696d7 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -526,7 +526,9 @@ static int stimer_set_config(struct kvm_vcpu_hv_stimer *stimer, u64 config,
 		new_config.enable = 0;
 	stimer->config.as_uint64 = new_config.as_uint64;
 
-	stimer_mark_pending(stimer, false);
+	if (stimer->config.enable)
+		stimer_mark_pending(stimer, false);
+
 	return 0;
 }
 
@@ -542,7 +544,10 @@ static int stimer_set_count(struct kvm_vcpu_hv_stimer *stimer, u64 count,
 		stimer->config.enable = 0;
 	else if (stimer->config.auto_enable)
 		stimer->config.enable = 1;
-	stimer_mark_pending(stimer, false);
+
+	if (stimer->config.enable)
+		stimer_mark_pending(stimer, false);
+
 	return 0;
 }
 
-- 
2.20.1