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

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Date:   Fri, 15 Mar 2019 19:04:04 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/kvm/hyper-v: avoid spurious pending stimer on vCPU
 init

On 13/03/19 18:13, 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@...hat.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;
>  }
>  
> 

Queued for after the merge window.

Paolo