linux-kernel - Re: [PART1 RFC v3 12/12] svm: Manage vcpu load/unload when enable AVIC

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Date:	Wed, 6 Apr 2016 14:36:27 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
Cc:	pbonzini@...hat.com, joro@...tes.org, bp@...en8.de,
	gleb@...nel.org, alex.williamson@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, wei@...hat.com,
	sherry.hurwitz@....com
Subject: Re: [PART1 RFC v3 12/12] svm: Manage vcpu load/unload when enable
 AVIC

2016-04-06 10:40+0700, Suravee Suthikulpanit:
> On 04/05/2016 09:56 PM, Radim Krčmář wrote:
>>I meant to change the place where we remember that is_running must not
>>be true.  Something like
>>
>>   svm_vcpu_blocking(struct kvm_vcpu *vcpu):
>>          vcpu->is_blocking = true;
>>          avic_set_running(vcpu, false);
>>
>>   avic_vcpu_load(struct kvm_vcpu *vcpu, bool is_load):
>>          avic_set_running(vcpu, is_load && !vcpu->is_blocking)
> 
> I assume that you also imply that we would also need:
> 
> 	svm_vcpu_unblocking(struct kvm_vcpu *vcpu) {
> 		avic_set_running(vcpu, false);
> 		vcpu->is_blocking = false;
> 	}

Yes, thought the order should be flipped in order to avoid suboptimal
case when preemption hits us after avic_set_running().

   static void svm_vcpu_unblocking(struct kvm_vcpu *vcpu) {
   	vcpu->is_blocking = false;
   	avic_set_running(vcpu, true);
   }

avic_set_running has barriers that prevent GCC from harmful reordering.