linux-kernel - Re: [RFC PATCH v2 7/7] KVM: KVM: don't break vcpu 'halt' state due to apfs

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Message-ID: <20101102065613.GA7881@redhat.com>
Date:	Tue, 2 Nov 2010 08:56:13 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Cc:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [RFC PATCH v2 7/7] KVM: KVM: don't break vcpu 'halt' state due
 to apfs

On Tue, Nov 02, 2010 at 10:30:10AM +0800, Xiao Guangrong wrote:
> On 11/01/2010 08:55 PM, Gleb Natapov wrote:
> 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 2cfdf2d..f7aed95 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -5295,8 +5295,9 @@ static int __vcpu_run(struct kvm_vcpu *vcpu)
> >  			{
> >  				switch(vcpu->arch.mp_state) {
> >  				case KVM_MP_STATE_HALTED:
> > -					vcpu->arch.mp_state =
> > -						KVM_MP_STATE_RUNNABLE;
> > +					if (list_empty_careful(&vcpu->async_pf.done))
> > +						vcpu->arch.mp_state =
> > +							KVM_MP_STATE_RUNNABLE;
> 
> if nmi/interrupt and apfs completed event occur at the same time, we will miss to
> exit halt sate. Maybe we can check the pending event here, see below patch please.
> 
No, we will not. If nmi/interrupt and apfs completed event occur at the same
time kvm_vcpu_block() will exit with KVM_REQ_UNHALT set, but cpu will
not be unhalted because of list_empty_careful(&vcpu->async_pf.done)
check. Vcpu then will process pending apf completion and enter
kvm_vcpu_block() again which will immediately exit because
kvm_arch_vcpu_runnable() will return true since there is pending
nmi/interrupt. This time vcpu will be unhalted.

> >  				case KVM_MP_STATE_RUNNABLE:
> >  					vcpu->arch.apf.halted = false;
> >  					break;
> > @@ -6279,6 +6280,7 @@ void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
> >  		vcpu->arch.fault.error_code = 0;
> >  		vcpu->arch.fault.address = work->arch.token;
> >  		kvm_inject_page_fault(vcpu);
> > +		vcpu->arch.mp_state = KVM_MP_STATE_RUNNABLE;
> >  	}
> >  }
> 
> Have a stupid question, why we make the vcpu runnable here? Sorry i don't know
> kvm pv guest to much. :-(
Because kvm_arch_vcpu_runnable() does not check for pending exceptions.
Since now we do not unhalt vcpu when apf completion happens we need to
unhalt it here. But, as I said, the patch is untested.

--
			Gleb.
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