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Date:   Tue, 12 Jan 2021 11:18:02 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Wei Huang <wei.huang2@....com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, vkuznets@...hat.com, joro@...tes.org,
        bp@...en8.de, tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
        jmattson@...gle.com, wanpengli@...cent.com, bsd@...hat.com,
        dgilbert@...hat.com, mlevitsk@...hat.com
Subject: Re: [PATCH 2/2] KVM: SVM: Add support for VMCB address check change

On Tue, Jan 12, 2021, Wei Huang wrote:
> New AMD CPUs have a change that checks VMEXIT intercept on special SVM
> instructions before checking their EAX against reserved memory region.
> This change is indicated by CPUID_0x8000000A_EDX[28]. If it is 1, KVM
> doesn't need to intercept and emulate #GP faults for such instructions
> because #GP isn't supposed to be triggered.
> 
> Co-developed-by: Bandan Das <bsd@...hat.com>
> Signed-off-by: Bandan Das <bsd@...hat.com>
> Signed-off-by: Wei Huang <wei.huang2@....com>
> ---
>  arch/x86/include/asm/cpufeatures.h | 1 +
>  arch/x86/kvm/svm/svm.c             | 2 +-
>  2 files changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 84b887825f12..ea89d6fdd79a 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -337,6 +337,7 @@
>  #define X86_FEATURE_AVIC		(15*32+13) /* Virtual Interrupt Controller */
>  #define X86_FEATURE_V_VMSAVE_VMLOAD	(15*32+15) /* Virtual VMSAVE VMLOAD */
>  #define X86_FEATURE_VGIF		(15*32+16) /* Virtual GIF */
> +#define X86_FEATURE_SVME_ADDR_CHK	(15*32+28) /* "" SVME addr check */

Heh, KVM should advertise this to userspace by setting the kvm_cpu_cap bit.  KVM
KVM forwards relevant VM-Exits to L1 without checking if rAX points at an
invalid L1 GPA.

>  /* Intel-defined CPU features, CPUID level 0x00000007:0 (ECX), word 16 */
>  #define X86_FEATURE_AVX512VBMI		(16*32+ 1) /* AVX512 Vector Bit Manipulation instructions*/
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 74620d32aa82..451b82df2eab 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -311,7 +311,7 @@ int svm_set_efer(struct kvm_vcpu *vcpu, u64 efer)
>  	svm->vmcb->save.efer = efer | EFER_SVME;
>  	vmcb_mark_dirty(svm->vmcb, VMCB_CR);
>  	/* Enable GP interception for SVM instructions if needed */
> -	if (efer & EFER_SVME)
> +	if ((efer & EFER_SVME) && !boot_cpu_has(X86_FEATURE_SVME_ADDR_CHK))
>  		set_exception_intercept(svm, GP_VECTOR);
>  
>  	return 0;
> -- 
> 2.27.0
> 

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