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

[Roman Kagan]

On Wed, Mar 13, 2019 at 06:13:42PM +0100, Vitaly Kuznetsov wrote:
> 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@xxxxxxxxxx>
> ---
>  arch/x86/kvm/hyperv.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)

Reviewed-by: Roman Kagan <rkagan@xxxxxxxxxxxxx>