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

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 f3b138c5d8 ("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: f3b138c5d8 ("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>
This commit is contained in:
Vitaly Kuznetsov 2019-03-13 18:13:42 +01:00 committed by Paolo Bonzini
parent 2bdb76c015
commit 013cc6ebbf
1 changed files with 7 additions and 2 deletions
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9
arch/x86/kvm/hyperv.c
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@ -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;
}