[PATCH v3 1/2] x86/cpufeatures: Add macros for Intel's new fast rep string features

[PATCH v3 1/2] x86/cpufeatures: Add macros for Intel's new fast rep string features

[Jim Mattson]

KVM_GET_SUPPORTED_CPUID should reflect these host CPUID bits. The bits
are already cached in word 12. Give the bits X86_FEATURE names, so
that they can be easily referenced. Hide these bits from
/proc/cpuinfo, since the host kernel makes no use of them at present.

Signed-off-by: Jim Mattson <jmattson@google.com>
---
 arch/x86/include/asm/cpufeatures.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index ef4775c6db01..454f0faa8e90 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -308,6 +308,9 @@
 /* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */
 #define X86_FEATURE_AVX_VNNI		(12*32+ 4) /* AVX VNNI instructions */
 #define X86_FEATURE_AVX512_BF16		(12*32+ 5) /* AVX512 BFLOAT16 instructions */
+#define X86_FEATURE_FZRM		(12*32+10) /* "" Fast zero-length REP MOVSB */
+#define X86_FEATURE_FSRS		(12*32+11) /* "" Fast short REP STOSB */
+#define X86_FEATURE_FSRC		(12*32+12) /* "" Fast short REP {CMPSB,SCASB} */
 
 /* AMD-defined CPU features, CPUID level 0x80000008 (EBX), word 13 */
 #define X86_FEATURE_CLZERO		(13*32+ 0) /* CLZERO instruction */
-- 
2.37.2.789.g6183377224-goog

[PATCH v3 2/2] KVM: x86: Expose CPUID.(EAX=7,ECX=1).EAX[12:10] to the guest

[Jim Mattson]

Fast zero-length REP MOVSB, fast short REP STOSB, and fast short REP
{CMPSB,SCASB} are inherent features of the processor that cannot be
hidden by the hypervisor. When these features are present on the host,
enumerate them in KVM_GET_SUPPORTED_CPUID.

Signed-off-by: Jim Mattson <jmattson@google.com>
---
 arch/x86/kvm/cpuid.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 75dcf7a72605..172cbfd60946 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -636,7 +636,7 @@ void kvm_set_cpu_caps(void)
 		kvm_cpu_cap_set(X86_FEATURE_SPEC_CTRL_SSBD);
 
 	kvm_cpu_cap_mask(CPUID_7_1_EAX,
-		F(AVX_VNNI) | F(AVX512_BF16)
+		F(AVX_VNNI) | F(AVX512_BF16) | F(FZRM) | F(FSRS) | F(FSRC)
 	);
 
 	kvm_cpu_cap_mask(CPUID_D_1_EAX,
-- 
2.37.2.789.g6183377224-goog

Re: [PATCH v3 2/2] KVM: x86: Expose CPUID.(EAX=7,ECX=1).EAX[12:10] to the guest

[Sean Christopherson]

The shortlog is trying to win some kind of award.

And technically, KVM_GET_SUPPORTED_CPUID advertises features to userspace, not
to the guest.  How about this?

  KVM: x86: Advertise fast REP string features inherent to the CPU

On Thu, Sep 01, 2022, Jim Mattson wrote:
> Fast zero-length REP MOVSB, fast short REP STOSB, and fast short REP
> {CMPSB,SCASB} are inherent features of the processor that cannot be
> hidden by the hypervisor. When these features are present on the host,
> enumerate them in KVM_GET_SUPPORTED_CPUID.
> 
> Signed-off-by: Jim Mattson <jmattson@google.com>

With a less cryptic shortlog,

Reviewed-by: Sean Christopherson <seanjc@google.com>

Re: [PATCH v3 1/2] x86/cpufeatures: Add macros for Intel's new fast rep string features

[H. Peter Anvin]

On 9/1/22 14:18, Jim Mattson wrote:
> KVM_GET_SUPPORTED_CPUID should reflect these host CPUID bits. The bits
> are already cached in word 12. Give the bits X86_FEATURE names, so
> that they can be easily referenced. Hide these bits from
> /proc/cpuinfo, since the host kernel makes no use of them at present.
> 
> Signed-off-by: Jim Mattson <jmattson@google.com>
> ---
>   arch/x86/include/asm/cpufeatures.h | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index ef4775c6db01..454f0faa8e90 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -308,6 +308,9 @@
>   /* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */
>   #define X86_FEATURE_AVX_VNNI		(12*32+ 4) /* AVX VNNI instructions */
>   #define X86_FEATURE_AVX512_BF16		(12*32+ 5) /* AVX512 BFLOAT16 instructions */
> +#define X86_FEATURE_FZRM		(12*32+10) /* "" Fast zero-length REP MOVSB */
> +#define X86_FEATURE_FSRS		(12*32+11) /* "" Fast short REP STOSB */
> +#define X86_FEATURE_FSRC		(12*32+12) /* "" Fast short REP {CMPSB,SCASB} */
>   
>   /* AMD-defined CPU features, CPUID level 0x80000008 (EBX), word 13 */
>   #define X86_FEATURE_CLZERO		(13*32+ 0) /* CLZERO instruction */

Any reason why these bits are hidden from /proc/cpuinfo?

	-hpa

Re: [PATCH v3 1/2] x86/cpufeatures: Add macros for Intel's new fast rep string features

[Borislav Petkov]

On Thu, Sep 01, 2022 at 09:14:24PM -0700, H. Peter Anvin wrote:
> Any reason why these bits are hidden from /proc/cpuinfo?

Yes, we aim to hide such purely CPUID bits from /proc/cpuinfo because it
becomes a dumping ground for "enablement" of new features. But

1. those features are not really used - most userspace like binutils and
gcc, etc do their own detection. (Yes, yes, I'd like to have ubiquitous
CPUID faulting).

2. /proc/cpuinfo is an ABI so we have to carry *all* those gazillion
flags for no good reason

So we have tools/arch/x86/kcpuid/ which we control and we can extend
with all the CPUID querying needs we have.

These kvm enablement things are kinda needed because guest userspace
gets an emulated CPUID so in order to detect features on its own, it
needs them. And kvm has tied features to x86's X86_FEATURE stuff and
there are sometimes weird interactions with it too but that's another
topic...

Oh and we still do add visible flags to /proc/cpuinfo but only when
they're features which need and have received non-trivial kernel
enablement like TDX or SNP or so.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

Re: [PATCH v3 1/2] x86/cpufeatures: Add macros for Intel's new fast rep string features

[Sean Christopherson]

On Thu, Sep 01, 2022, Jim Mattson wrote:
> KVM_GET_SUPPORTED_CPUID should reflect these host CPUID bits. The bits
> are already cached in word 12. Give the bits X86_FEATURE names, so
> that they can be easily referenced. Hide these bits from
> /proc/cpuinfo, since the host kernel makes no use of them at present.
> 
> Signed-off-by: Jim Mattson <jmattson@google.com>
> ---

Reviewed-by: Sean Christopherson <seanjc@google.com>

Re: [PATCH v3 1/2] x86/cpufeatures: Add macros for Intel's new fast rep string features

[Sean Christopherson]

On Thu, 01 Sep 2022 14:18:06 -0700, Jim Mattson wrote:
> KVM_GET_SUPPORTED_CPUID should reflect these host CPUID bits. The bits
> are already cached in word 12. Give the bits X86_FEATURE names, so
> that they can be easily referenced. Hide these bits from
> /proc/cpuinfo, since the host kernel makes no use of them at present.
> 
> 

Applied to kvm-x86 misc, thanks!

[1/2] x86/cpufeatures: Add macros for Intel's new fast rep string features
      https://github.com/kvm-x86/linux/commit/751b1e1ee8e9
[2/2] KVM: x86: Expose CPUID.(EAX=7,ECX=1).EAX[12:10] to the guest
      https://github.com/kvm-x86/linux/commit/bbbe171ac12f

--
https://github.com/kvm-x86/linux/tree/next
https://github.com/kvm-x86/linux/tree/fixes