linux-kernel - Re: [PATCH 13/14] x86: BHI stubs

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Message-ID: <20240930213030.ixbsyzziy6frh62f@treble>
Date: Mon, 30 Sep 2024 14:30:30 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, alyssa.milburn@...el.com,
	scott.d.constable@...el.com, joao@...rdrivepizza.com,
	andrew.cooper3@...rix.com, jose.marchesi@...cle.com,
	hjl.tools@...il.com, ndesaulniers@...gle.com,
	samitolvanen@...gle.com, nathan@...nel.org, ojeda@...nel.org,
	kees@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH 13/14] x86: BHI stubs

On Fri, Sep 27, 2024 at 09:49:09PM +0200, Peter Zijlstra wrote:
> +/*
> + * At the function start, launder function arguments that are a pointer through
> + * CMOVcc, this will create a write dependency in the speculation flow.
> + *
> + * Notably, the CFI preambles calling these will have ZF set and r10 zero.
> + */
> +
> +.pushsection .noinstr.text, "ax"
> +
> +	.align 8
> +SYM_CODE_START(__bhi_args_6c1)
> +	ANNOTATE_NOENDBR
> +	.align 8
> +SYM_INNER_LABEL(__bhi_args_0, SYM_L_LOCAL)
> +	UNWIND_HINT_FUNC
> +	cmovne %r10, %rdi

IIUC, this works because if the "jz" in the CFI preamble mispredicts to
the __bhi_args_* code, "cmovne" will zero out the speculative value of
rdi.

Why use %r10 instead of a literal $0?  Also how do you know %r10 is 0?

-- 
Josh