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Date: Tue, 24 Oct 2023 12:36:01 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
	"H. Peter Anvin" <hpa@zytor.com>,
	Josh Poimboeuf <jpoimboe@kernel.org>,
	Andy Lutomirski <luto@kernel.org>, Jonathan Corbet <corbet@lwn.net>,
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	Daniel Sneddon <daniel.sneddon@linux.intel.com>,
	antonio.gomez.iglesias@linux.intel.com,
	Alyssa Milburn <alyssa.milburn@intel.com>
Subject: Re: [PATCH  v2 1/6] x86/bugs: Add asm helpers for executing VERW
Message-ID: <20231024103601.GH31411@noisy.programming.kicks-ass.net>
References: <20231024-delay-verw-v2-0-f1881340c807@linux.intel.com>
 <20231024-delay-verw-v2-1-f1881340c807@linux.intel.com>
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In-Reply-To: <20231024-delay-verw-v2-1-f1881340c807@linux.intel.com>

On Tue, Oct 24, 2023 at 01:08:21AM -0700, Pawan Gupta wrote:

> +.macro CLEAR_CPU_BUFFERS
> +	ALTERNATIVE "jmp .Lskip_verw_\@;", "jmp .Ldo_verw_\@", X86_FEATURE_CLEAR_CPU_BUF
> +		/* nopl __KERNEL_DS(%rax) */
> +		.byte 0x0f, 0x1f, 0x80, 0x00, 0x00;
> +.Lverw_arg_\@:	.word __KERNEL_DS;
> +.Ldo_verw_\@:	verw _ASM_RIP(.Lverw_arg_\@);
> +.Lskip_verw_\@:
> +.endm

Why can't this be:

	ALTERNATIVE "". "verw _ASM_RIP(mds_verw_sel)", X86_FEATURE_CLEAR_CPU_BUF

And have that mds_verw_sel thing be out-of-line ? That gives much better
code for the case where we don't need this.