linux-kernel - Re: [PATCH] KVM: LAPIC: Accurately guarantee busy wait for timer to expire when using hv

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Date:   Thu, 29 Apr 2021 17:40:20 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: LAPIC: Accurately guarantee busy wait for timer to
 expire when using hv_timer

On 29/04/21 00:56, Sean Christopherson wrote:
> On Wed, Apr 28, 2021, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpengli@...cent.com>
>>
>> Commit ee66e453db13d (KVM: lapic: Busy wait for timer to expire when
>> using hv_timer) tries to set ktime->expired_tscdeadline by checking
>> ktime->hv_timer_in_use since lapic timer oneshot/periodic modes which
>> are emulated by vmx preemption timer also get advanced, they leverage
>> the same vmx preemption timer logic with tsc-deadline mode. However,
>> ktime->hv_timer_in_use is cleared before apic_timer_expired() handling,
>> let's delay this clearing in preemption-disabled region.
>>
>> Fixes: ee66e453db13d (KVM: lapic: Busy wait for timer to expire when using hv_timer)
> 
> Well that's embarassing.  I suspect/hope I tested the case where start_hv_timer()
> detects the timer already expired.  On the plus side, start_hv_timer() calls
> cancel_hv_timer() after apic_timer_expired(), so there are unlikely to be hidden
> side effects (and cancel_hv_timer() is tiny).
> 
>> Cc: Sean Christopherson <seanjc@...gle.com>
>> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> 
> Reviewed-by: Sean Christopherson <seanjc@...gle.com>
> 
>> ---
>>   arch/x86/kvm/lapic.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
>> index 152591f..c0ebef5 100644
>> --- a/arch/x86/kvm/lapic.c
>> +++ b/arch/x86/kvm/lapic.c
>> @@ -1913,8 +1913,8 @@ void kvm_lapic_expired_hv_timer(struct kvm_vcpu *vcpu)
>>   	if (!apic->lapic_timer.hv_timer_in_use)
>>   		goto out;
>>   	WARN_ON(rcuwait_active(&vcpu->wait));
>> -	cancel_hv_timer(apic);
>>   	apic_timer_expired(apic, false);
>> +	cancel_hv_timer(apic);
>>   
>>   	if (apic_lvtt_period(apic) && apic->lapic_timer.period) {
>>   		advance_periodic_target_expiration(apic);
>> -- 
>> 2.7.4
>>
> 

Queued, thanks.

Paolo