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

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Message-ID: <20250930083137.GH3245006@noisy.programming.kicks-ass.net>
Date: Tue, 30 Sep 2025 10:31:37 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Juergen 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 Tue, Sep 30, 2025 at 09:03:53AM +0200, Juergen Gross wrote:

> +static __always_inline bool __wrmsrq_constant(u32 msr, u64 val, int type)
> +{
> +	BUILD_BUG_ON(!__builtin_constant_p(msr));
> +
> +	asm_inline volatile goto(
> +		"1:\n"
> +		ALTERNATIVE_2(PREPARE_RCX_RDX_FOR_WRMSR
> +			      "2: ds wrmsr",
> +			      PREPARE_RCX_RDX_FOR_WRMSR
> +			      ASM_WRMSRNS,
> +			      X86_FEATURE_WRMSRNS,
> +			      ASM_WRMSRNS_IMM,
> +			      X86_FEATURE_MSR_IMM)
> +		_ASM_EXTABLE_TYPE(1b, %l[badmsr], %c[type])	/* For WRMSRNS immediate */
> +		_ASM_EXTABLE_TYPE(2b, %l[badmsr], %c[type])	/* For WRMSR(NS) */
> +
> +		:
> +		: [val] "a" (val), [msr] "i" (msr), [type] "i" (type)
> +		: "memory", "ecx", "rdx"
> +		: badmsr);
> +
> +	return false;
> +
> +badmsr:
> +	return true;
> +}

Just wondering, would something this work?

	asm_inline volatile goto(
		"1:\n"
		ALTERNATIVE(PREPARE_RCX_RDX_FOR_WRMSR
			    "2:\n"
			    ALTERNATIVE("ds wrmsr",
			                ASM_WRMSRNS, X86_FEATURE_WRMSRNS),
			    ASM_WRMSRNS_IMM, X86_FEATURE_MSR_IMM);
		_ASM_EXTABLE_TYPE(1b, %l[badmsr], %c[type])	/* For WRMSRNS immediate */
		_ASM_EXTABLE_TYPE(2b, %l[badmsr], %c[type])	/* For WRMSR(NS) */

		:
		: [val] "a" (val), [msr] "i" (msr), [type] "i" (type)
		: "memory", "ecx", "rdx"
		: badmsr);

Its a bit weird because the nested alternative isn't for the exact same
position I suppose. But I find it a more readable form.