Re: [PATCH v2 2/2] KVM: x86: Expose Predictive Store Forwarding Disable on Intel parts

[Sean Christopherson <seanjc@google.com>]

On Tue, Aug 30, 2022, Jim Mattson wrote:
> Intel enumerates support for PSFD in CPUID.(EAX=7,ECX=2):EDX.PSFD[bit
> 0]. Report support for this feature in KVM if it is available on the
> host.
> 
> Presumably, this feature bit, like its AMD counterpart, is not welcome
> in cpufeatures.h, so add a local definition of this feature in KVM.
> 
> Signed-off-by: Jim Mattson <jmattson@google.com>
> ---
>  arch/x86/kvm/cpuid.c | 23 +++++++++++++++++------
>  1 file changed, 17 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 07be45c5bb93..b5af9e451bef 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -62,6 +62,7 @@ u32 xstate_required_size(u64 xstate_bv, bool compacted)
>   * This one is tied to SSB in the user API, and not
>   * visible in /proc/cpuinfo.
>   */
> +#define KVM_X86_FEATURE_PSFD		0          /* Predictive Store Forwarding Disable */

I believe we can use "enum kvm_only_cpuid_leafs" to handle this.  E.g. 

	enum kvm_only_cpuid_leafs {
		CPUID_12_EAX	 = NCAPINTS,
		CPUID_7_2_EDX,
		NR_KVM_CPU_CAPS,

		NKVMCAPINTS = NR_KVM_CPU_CAPS - NCAPINTS,
	};

then the intended use of KVM_X86_FEATURE_*

	#define KVM_X86_FEATURE_PSFD	KVM_X86_FEATURE(CPUID_7_2_EDX, 0)

I _think_ we can then define an arbitrary word for X86_FEATURE_PSFD, e.g.

	#define X86_FEATURE_PSFD	(NKVMCAPINTS*32+0)

and then wire up the translation:

	static __always_inline u32 __feature_translate(int x86_feature)
	{
		if (x86_feature == X86_FEATURE_SGX1)
			return KVM_X86_FEATURE_SGX1;
		else if (x86_feature == X86_FEATURE_SGX2)
			return KVM_X86_FEATURE_SGX2;
		else if (x86_feature == X86_FEATURE_PSFD)
			return KVM_X86_FEATURE_PSFD;
	
		return x86_feature;
	}

I believe/hope that allows us to use at least cpuid_entry_override().  Open coding
masking of specific registers was a mess that I don't want to repeat.