linux-kernel - Re: [PATCH 18/19] KVM: SVM: Ignore writes to Remote Read Data on AVIC write traps

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Date:   Wed, 31 Aug 2022 13:40:30 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Li RongQing <lirongqing@...du.com>
Subject: Re: [PATCH 18/19] KVM: SVM: Ignore writes to Remote Read Data on
 AVIC write traps

On Wed, 2022-08-31 at 00:35 +0000, Sean Christopherson wrote:
> Drop writes to APIC_RRR, a.k.a. Remote Read Data Register, on AVIC
> unaccelerated write traps.  The register is read-only and isn't emulated
> by KVM.  Sending the register through kvm_apic_write_nodecode() will
> result in screaming when x2APIC is enabled due to the unexpected failure
> to retrieve the MSR (KVM expects that only "legal" accesses will trap).

I wonder about ESR register as well (280H), KVM doesn't seem to support it either,
but allows 0 writes. Anyway:

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky

> 
> Fixes: 4d1d7942e36a ("KVM: SVM: Introduce logic to (de)activate x2AVIC mode")
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/svm/avic.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index dad5affe44c1..b2033a56010c 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -675,6 +675,9 @@ static int avic_unaccel_trap_write(struct kvm_vcpu *vcpu)
>  	case APIC_DFR:
>  		avic_handle_dfr_update(vcpu);
>  		break;
> +	case APIC_RRR:
> +		/* Ignore writes to Read Remote Data, it's read-only. */
> +		return 1;
>  	default:
>  		break;
>  	}