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

[Sean Christopherson <seanjc@google.com>]

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},
 };