linux-kernel - Re: [PATCH v2 09/12] x86/msr: Use the alternatives mechanism for WRMSR

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Message-ID: <4c4cce9f-3219-48f0-8606-6573339b8794@intel.com>
Date: Fri, 3 Oct 2025 07:23:29 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Peter Zijlstra <peterz@...radead.org>, Jürgen Groß
 <jgross@...e.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, llvm@...ts.linux.dev,
 xin@...or.com, "H. Peter Anvin" <hpa@...or.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Nathan Chancellor <nathan@...nel.org>,
 Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>
Subject: Re: [PATCH v2 09/12] x86/msr: Use the alternatives mechanism for
 WRMSR

On 10/1/25 00:23, Peter Zijlstra wrote:
> On Wed, Oct 01, 2025 at 08:43:39AM +0200, Peter Zijlstra wrote:
>> Let me see how terrible it all ends up when using as macros
> Argh, as macros are differently painful. I hate computers :/

ALTERNATIVES are fun and all, but is there a good reason we're pulling
out our hair to use them here?

Normal WRMSR is slooooooooooow. The ones that aren't slow don't need
WRMSRNS in the first place.

Would an out-of-line wrmsr() with an if() in it be so bad? Or a
static_call()? Having WRMSR be inlined in a laudable goal, but I'm
really asking if it's worth it.