Re: [PATCH v12 01/37] x86/cpufeatures: Add the cpu feature bit for WRMSRNS

Borislav Petkov
2023-11-08T12:37:23Z

On Mon, Oct 02, 2023 at 11:24:22PM -0700, Xin Li wrote:
> Subject: Re: [PATCH v12 01/37] x86/cpufeatures: Add the cpu feature bit for WRMSRNS
^^^^

For all your text:

s/cpu/CPU/g

> WRMSRNS is an instruction that behaves exactly like WRMSR, with
> the only difference being that it is not a serializing instruction
> by default. Under certain conditions, WRMSRNS may replace WRMSR to
> improve performance.
>
> Add the CPU feature bit for WRMSRNS.
>
> Tested-by: Shan Kang <shan.kang@intel.com>
> Signed-off-by: Xin Li <xin3.li@intel.com>
> ---
> arch/x86/include/asm/cpufeatures.h | 1 +
> tools/arch/x86/include/asm/cpufeatures.h | 1 +
> 2 files changed, 2 insertions(+)

It looks to me like you can merge the first three patches into one as
all they do is add that insn support.

Then, further down in the patchset, it says:

+ if (cpu_feature_enabled(X86_FEATURE_FRED)) {
+ /* WRMSRNS is a baseline feature for FRED. */

but WRMSRNS is not mentioned in the FRED spec "Document Number:
346446-005US, Revision: 5.0" which, according to

https://www.intel.com/content/www/us/en/content-details/780121/flexible-return-and-event-delivery-fred-specification.html

is the latest.

Am I looking at the wrong one?

> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 58cb9495e40f..330876d34b68 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -322,6 +322,7 @@
> #define X86_FEATURE_FSRS (12*32+11) /* "" Fast short REP STOSB */
> #define X86_FEATURE_FSRC (12*32+12) /* "" Fast short REP {CMPSB,SCASB} */
> #define X86_FEATURE_LKGS (12*32+18) /* "" Load "kernel" (userspace) GS */
> +#define X86_FEATURE_WRMSRNS (12*32+19) /* "" Non-Serializing Write to Model Specific Register instruction */

/* "" Non-serializing WRMSR */

is more than enough.

And now I'm wondering: when you're adding a separate CPUID bit, then the
above should be

+ if (cpu_feature_enabled(X86_FEATURE_WRMSRNS)) {
+ /* WRMSRNS is a baseline feature for FRED. */

I see that you're adding a dependency:

+ { X86_FEATURE_FRED, X86_FEATURE_WRMSRNS },

which then means you don't need the X86_FEATURE_WRMSRNS definition at
all and can use X86_FEATURE_FRED only.

So, what's up?

Thx.

--
Regards/Gruss,
Boris.