linux-kernel - Re: [PATCH v3 3/4] KVM: SVM: Add support for SVM instruction address check change

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Date:   Tue, 26 Jan 2021 13:52:50 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Wei Huang <wei.huang2@....com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, pbonzini@...hat.com,
        vkuznets@...hat.com, seanjc@...gle.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, luto@...capital.net
Subject: Re: [PATCH v3 3/4] KVM: SVM: Add support for SVM instruction
 address check change

On Tue, 2021-01-26 at 03:18 -0500, 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, #VMEXIT
> is triggered before #GP. KVM doesn't need to intercept and emulate #GP
> faults as #GP is 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>
> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
> ---
>  arch/x86/include/asm/cpufeatures.h | 1 +
>  arch/x86/kvm/svm/svm.c             | 3 +++
>  2 files changed, 4 insertions(+)
> 
> 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 */
>  
>  /* 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 e5ca01e25e89..f9233c79265b 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1036,6 +1036,9 @@ static __init int svm_hardware_setup(void)
>  		}
>  	}
>  
> +	if (boot_cpu_has(X86_FEATURE_SVME_ADDR_CHK))
> +		svm_gp_erratum_intercept = false;
> +
Again, I would make svm_gp_erratum_intercept a tri-state module param,
and here if it is in 'auto' state do this.

Also I might as well made this code fail if X86_FEATURE_SVME_ADDR_CHK is set but
user insists on svm_gp_erratum_intercept = true.

>  	if (vgif) {
>  		if (!boot_cpu_has(X86_FEATURE_VGIF))
>  			vgif = false;


Best regards,
	Maxim Levitsky