linux-kernel - Re: [PATCH] svm: Fix improper check when deactivate AVIC

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Date:   Fri, 1 Feb 2019 11:01:22 +0000
From:   "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
CC:     "joro@...tes.org" <joro@...tes.org>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "hpa@...or.com" <hpa@...or.com>
Subject: Re: [PATCH] svm: Fix improper check when deactivate AVIC

Paolo,

On 1/30/19 11:22 PM, Paolo Bonzini wrote:
> On 29/01/19 09:09, Suthikulpanit, Suravee wrote:
>> The function svm_refresh_apicv_exec_ctrl() always returning prematurely
>> as kvm_vcpu_apicv_active() always return false when calling from
>> the function arch/x86/kvm/x86.c:kvm_vcpu_deactivate_apicv().
>> This is because the apicv_active is set to false just before calling
>> refresh_apicv_exec_ctrl().
>>
>> Also, we need to mark VMCB_AVIC bit as dirty instead of VMCB_INTR.
>>
>> So, fix svm_refresh_apicv_exec_ctrl() to properly deactivate AVIC.
>>
>> Fixes: 67034bb9dd5e ('KVM: SVM: Add irqchip_split() checks before enabling AVIC')
> 
> Having the lapic_in_kernel check in your code is a bit ugly - the caller
> should check it.  And actually, even though it doesn't check it right
> now, svm_refresh_apicv_exec_ctrl is only reachable if lapic_in_kernel
> returns true; see the KVM_CAP_HYPERV_SYNIC case of
> kvm_vcpu_ioctl_enable_cap.
> 
> What about this simpler patch:
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 01b66bbcdd7a..74ceda470eae 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -5122,11 +5122,11 @@ static void svm_refresh_apicv_exec_ctrl(
>   	struct vcpu_svm *svm = to_svm(vcpu);
>   	struct vmcb *vmcb = svm->vmcb;
> 
> -	if (!kvm_vcpu_apicv_active(&svm->vcpu))
> -		return;
> -
> -	vmcb->control.int_ctl &= ~AVIC_ENABLE_MASK;
> -	mark_dirty(vmcb, VMCB_INTR);
> +	if (kvm_vcpu_apicv_active(vcpu))
> +		vmcb->control.int_ctl |= AVIC_ENABLE_MASK;
> +	else
> +		vmcb->control.int_ctl &= ~AVIC_ENABLE_MASK;
> +	mark_dirty(vmcb, VMCB_AVIC);
>   }
> 
>   static void svm_load_eoi_exitmap(struct kvm_vcpu *vcpu, u64
> *eoi_exit_bitmap)
> 
> ?
> 
> Thanks,
> 
> Paolo
> 

Yes, this would work.

This patch is actually a precursor of a patch series that I am working on
to allow AVIC enabling/disabling during run-time. I will send out a series
for RFC soon.

Thanks,
Suravee