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

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Message-ID: <20231108123647.GBZUuA31zntox0W0gu@fat_crate.local>
Date:   Wed, 8 Nov 2023 13:36:47 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Xin Li <xin3.li@...el.com>
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-edac@...r.kernel.org, linux-hyperv@...r.kernel.org,
        kvm@...r.kernel.org, xen-devel@...ts.xenproject.org,
        tglx@...utronix.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, luto@...nel.org,
        pbonzini@...hat.com, seanjc@...gle.com, peterz@...radead.org,
        jgross@...e.com, ravi.v.shankar@...el.com, mhiramat@...nel.org,
        andrew.cooper3@...rix.com, jiangshanlai@...il.com,
        nik.borisov@...e.com
Subject: Re: [PATCH v12 01/37] x86/cpufeatures: Add the cpu feature bit for
 WRMSRNS

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@...el.com>
> Signed-off-by: Xin Li <xin3.li@...el.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.

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