linux-kernel - Re: [PATCH v1 2/4] x86/fred: Write to FRED MSRs with wrmsrns()

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Message-ID: <20240705092805.GC11386@noisy.programming.kicks-ass.net>
Date: Fri, 5 Jul 2024 11:28:05 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: Borislav Petkov <bp@...en8.de>, dave.hansen@...el.com, xin@...or.com,
	linux-kernel@...r.kernel.org, hpa@...or.com, tglx@...utronix.de,
	mingo@...hat.com, dave.hansen@...ux.intel.com, x86@...nel.org,
	nik.borisov@...e.com, houwenlong.hwl@...group.com
Subject: Re: [PATCH v1 2/4] x86/fred: Write to FRED MSRs with wrmsrns()

On Wed, Jul 03, 2024 at 05:00:53PM +0100, Andrew Cooper wrote:

> /* Non-serialising WRMSR, when available.  Falls back to a serialising WRMSR. */
> static inline void wrmsrns(uint32_t msr, uint32_t lo, uint32_t hi)
> {
>     /*
>      * WRMSR is 2 bytes.  WRMSRNS is 3 bytes.  Pad WRMSR with a redundant CS
>      * prefix to avoid a trailing NOP.
>      */
>     alternative_input(".byte 0x2e; wrmsr",
>                       ".byte 0x0f,0x01,0xc6", X86_FEATURE_WRMSRNS,
>                       "c" (msr), "a" (lo), "d" (hi));
> }

FWIW, I favour this variant.