linux-kernel - Re: [PATCH RESEND] KVM: Boosting vCPUs that are delivering interrupts

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Message-ID: <e31024e4-f437-becd-a9e3-e1ea8cd2e0c7@redhat.com>
Date:   Thu, 18 Jul 2019 10:34:04 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>,
        Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH RESEND] KVM: Boosting vCPUs that are delivering interrupts

On 18/07/19 10:15, Christian Borntraeger wrote:
> 
> 
> On 18.07.19 09:59, Paolo Bonzini wrote:
>> On 12/07/19 09:15, Wanpeng Li wrote:
>>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>>> index b4ab59d..2c46705 100644
>>> --- a/virt/kvm/kvm_main.c
>>> +++ b/virt/kvm/kvm_main.c
>>> @@ -2404,8 +2404,10 @@ void kvm_vcpu_kick(struct kvm_vcpu *vcpu)
>>>  	int me;
>>>  	int cpu = vcpu->cpu;
>>>  
>>> -	if (kvm_vcpu_wake_up(vcpu))
>>> +	if (kvm_vcpu_wake_up(vcpu)) {
>>> +		vcpu->preempted = true;
>>>  		return;
>>> +	}
>>>  
>>>  	me = get_cpu();
>>>  	if (cpu != me && (unsigned)cpu < nr_cpu_ids && cpu_online(cpu))
>>>
>>
>> Who is resetting vcpu->preempted to false in this case?  This also
>> applies to s390 in fact.
> 
> Isnt that done by the sched_in handler?

I am a bit confused because, if it is done by the sched_in later, I
don't understand why the sched_out handler hasn't set vcpu->preempted
already.

The s390 commit message is not very clear, but it talks about "a former
sleeping cpu" that "gave up the cpu voluntarily".  Does "voluntarily"
that mean it is in kvm_vcpu_block?  But then at least for x86 it would
be after vcpu_load so the preempt notifiers have been registered, and
for s390 too (kvm_arch_vcpu_ioctl_run -> __vcpu_run -> vcpu_post_run ->
kvm_handle_sie_intercept etc.).

Paolo