Re: [PATCH 2/2] KVM: SVM: Add support for VMCB address check change

Maxim Levitsky
2021-01-14T12:06:11Z

On Tue, 2021-01-12 at 00:37 -0600, 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@redhat.com>
> Signed-off-by: Bandan Das <bsd@redhat.com>
> Signed-off-by: Wei Huang <wei.huang2@amd.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 */
Why ""?

>
> /* 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);

As mentioned in the review for the other patch I would add a flag that
would enable the workaround for the errata, and I would force it disabled
if X86_FEATURE_SVME_ADDR_CHK is set in CPUID somewhere early in the
kvm initialization.

And finally that new flag can be used here to enable the #GP interception
in the above code.

>
> return 0;

Best regards,
Maxim Levitsky