Re: [PATCH 4/6] x86: KVM: Enable AVX-VNNI-INT8 CPUID and expose it to guest

Sean Christopherson
2022-10-19T15:17:56Z

On Wed, Oct 19, 2022, Sean Christopherson wrote:
> On Wed, Oct 19, 2022, Borislav Petkov wrote:
> > On Wed, Oct 19, 2022 at 04:47:32PM +0800, Jiaxi Chen wrote:
> > > AVX-VNNI-INT8 is a new set of instructions in the latest Intel platform
> > > Sierra Forest. It multiplies the individual bytes of two unsigned or
> > > unsigned source operands, then add and accumulate the results into the
> > > destination dword element size operand. This instruction allows for the
> > > platform to have superior AI capabilities.
> > >
> > > The bit definition:
> > > CPUID.(EAX=7,ECX=1):EDX[bit 4]
> >
> > For this particular one, use scattered.c instead of adding a new leaf.
>
> Unless the kernel wants to use X86_FEATURE_AVX_VNNI_INT8, which seems unlikely,
> there's no need to create a scattered entry. This can be handled in KVM by adding
> a KVM-only leaf entry (which will be needed no matter what), plus a #define for
> X86_FEATURE_AVX_VNNI_INT8 to direct it to the KVM entry.
>
> E.g.
>
> diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
> index a19d473d0184..25e7bfc61607 100644
> --- a/arch/x86/kvm/reverse_cpuid.h
> +++ b/arch/x86/kvm/reverse_cpuid.h
> @@ -13,6 +13,7 @@
> */
> enum kvm_only_cpuid_leafs {
> CPUID_12_EAX = NCAPINTS,
> + CPUID_7_1_EDX,
> NR_KVM_CPU_CAPS,
>
> NKVMCAPINTS = NR_KVM_CPU_CAPS - NCAPINTS,
> @@ -24,6 +25,16 @@ enum kvm_only_cpuid_leafs {
> #define KVM_X86_FEATURE_SGX1 KVM_X86_FEATURE(CPUID_12_EAX, 0)
> #define KVM_X86_FEATURE_SGX2 KVM_X86_FEATURE(CPUID_12_EAX, 1)
>
> +#define KVM_X86_FEATURE_AVX_VNNI_INT8 KVM_X86_FEATURE(CPUID_7_1_EDX, 4)
> +
> +/*
> + * Alias X86_FEATURE_* to the KVM variant for features in KVM-only leafs that
> + * aren't scattered by cpufeatures.h so that X86_FEATURE_* can be used in KVM,
> + * e.g. to query guest CPUID. As a bonus, no translation is needed for these
> + * features in __feature_translate().
> + */
> +#define X86_FEATURE_AVX_VNNI_INT8 KVM_X86_FEATURE_AVX_VNNI_INT8

Actually, there's no need for KVM_X86_FEATURE_AVX_VNNI_INT8 in this case, just
#define X86_FEATURE_AVX_VNNI_INT8 directly. The KVM_ prefixed macro exists purely
to redirect the non-KVM_ version, but that's unnecessary in this case.

diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
index a19d473d0184..38adafb03490 100644
--- a/arch/x86/kvm/reverse_cpuid.h
+++ b/arch/x86/kvm/reverse_cpuid.h
@@ -13,6 +13,7 @@
*/
enum kvm_only_cpuid_leafs {
CPUID_12_EAX = NCAPINTS,
+ CPUID_7_1_EDX,
NR_KVM_CPU_CAPS,

NKVMCAPINTS = NR_KVM_CPU_CAPS - NCAPINTS,
@@ -24,6 +25,13 @@ enum kvm_only_cpuid_leafs {
#define KVM_X86_FEATURE_SGX1 KVM_X86_FEATURE(CPUID_12_EAX, 0)
#define KVM_X86_FEATURE_SGX2 KVM_X86_FEATURE(CPUID_12_EAX, 1)

+/*
+ * Omit the KVM_ prefix for features in KVM-only leafs that aren't scattered by
+ * cpufeatures.h so that X86_FEATURE_* can be used in KVM,* e.g. to query guest
+ * CPUID. As a bonus, no handling in __feature_translate() is needed.
+ */
+#define X86_FEATURE_AVX_VNNI_INT8 KVM_X86_FEATURE(CPUID_7_1_EDX, 4)
+
struct cpuid_reg {
u32 function;
u32 index;
@@ -48,6 +56,7 @@ static const struct cpuid_reg reverse_cpuid[] = {
[CPUID_7_1_EAX] = { 7, 1, CPUID_EAX},
[CPUID_12_EAX] = {0x00000012, 0, CPUID_EAX},
[CPUID_8000_001F_EAX] = {0x8000001f, 0, CPUID_EAX},
+ [CPUID_7_1_EDX] = { 7, 1, CPUID_EDX},
};