From: Vitaly Kuznetsov <vkuznets@redhat.com>
Date: Wed, 13 Mar 2019 18:13:42 +0100
Subject: x86/kvm/hyper-v: avoid spurious pending stimer on vCPU init
Git-commit: 013cc6ebbf41496ce4badedd71ea6d4a6d198c14
Patch-mainline: v5.1-rc3
References: bsc#1132572

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>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Joerg Roedel <jroedel@suse.de>
---
 arch/x86/kvm/hyperv.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -527,7 +527,10 @@ static int stimer_set_config(struct kvm_
 	if ((stimer->config & HV_STIMER_ENABLE) && HV_STIMER_SINT(config) == 0)
 		config &= ~HV_STIMER_ENABLE;
 	stimer->config = config;
-	stimer_mark_pending(stimer, false);
+
+	if (stimer->config & HV_STIMER_ENABLE)
+		stimer_mark_pending(stimer, false);
+
 	return 0;
 }
 
@@ -543,7 +546,10 @@ static int stimer_set_count(struct kvm_v
 		stimer->config &= ~HV_STIMER_ENABLE;
 	else if (stimer->config & HV_STIMER_AUTOENABLE)
 		stimer->config |= HV_STIMER_ENABLE;
-	stimer_mark_pending(stimer, false);
+
+	if (stimer->config & HV_STIMER_ENABLE)
+		stimer_mark_pending(stimer, false);
+
 	return 0;
 }