Re: [PATCH v4 4/6] x86: KVM: Advertise AVX-VNNI-INT8 CPUID to user space

[Sean Christopherson]

 On Fri, Nov 18, 2022, Jiaxi Chen wrote:
> AVX-VNNI-INT8 is a new set of instructions in the latest Intel platform
> Sierra Forest, aims for the platform to have superior AI capabilities.
> This instruction multiplies the individual bytes of two unsigned or
> unsigned source operands, then adds and accumulates the results into the
> destination dword element size operand.
> 
> The bit definition:
> CPUID.(EAX=7,ECX=1):EDX[bit 4]
> 
> This CPUID is exposed to user space. Besides, there is no other VMX
> control for this instruction.
> 
> Signed-off-by: Jiaxi Chen <jiaxi.chen@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kvm/cpuid.c         | 5 ++++-
>  arch/x86/kvm/reverse_cpuid.h | 5 +++++
>  2 files changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 5726afb2d14c..e2b8e5485474 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -660,6 +660,9 @@ void kvm_set_cpu_caps(void)
>  		F(AVX_VNNI) | F(AVX512_BF16) | F(CMPCCXADD) | F(AMX_FP16) |
>  		F(AVX_IFMA));
>  
> +	kvm_cpu_cap_init_scattered(CPUID_7_1_EDX,

Ah, this is going to be confusing and potentially error prone.  AVX_VNNI_INT8
isn't actually scattered, i.e. kvm_cpu_cap_init_scattered() is poorly named.  And
using SF() would be _really_ broken as boot_cpu_has() would consume garbage and
potentially leak kernel state to userspace.

To address these issue and also document how to add KVM-only features, can you
slot in the two attached patches at the begining of this series?

Thanks!

> +		F(AVX_VNNI_INT8));

Terminators on a separate line please.

>  	kvm_cpu_cap_mask(CPUID_D_1_EAX,
>  		F(XSAVEOPT) | F(XSAVEC) | F(XGETBV1) | F(XSAVES) | f_xfd
>  	);