[v1] KVM: x86: AVIC and local APIC fixes+cleanups

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

Posted by Sean Christopherson 3 years, 7 months ago

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).

Fixes: 4d1d7942e36a ("KVM: SVM: Introduce logic to (de)activate x2AVIC mode")
Signed-off-by: Sean Christopherson <seanjc@google.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;
 	}
-- 
2.37.2.672.g94769d06f0-goog

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

Posted by Maxim Levitsky 3 years, 7 months ago

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@redhat.com>

Best regards,
	Maxim Levitsky

> 
> Fixes: 4d1d7942e36a ("KVM: SVM: Introduce logic to (de)activate x2AVIC mode")
> Signed-off-by: Sean Christopherson <seanjc@google.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;
>  	}