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

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Message-ID: <20240930223848.ulipiky3uw52ej56@treble>
Date: Mon, 30 Sep 2024 15:38:48 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org, alyssa.milburn@...el.com,
	scott.d.constable@...el.com, joao@...rdrivepizza.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 Mon, Sep 30, 2024 at 11:23:38PM +0100, Andrew Cooper wrote:
> On 30/09/2024 10:30 pm, Josh Poimboeuf wrote:
> > On Fri, Sep 27, 2024 at 09:49:09PM +0200, Peter Zijlstra wrote:
> >> +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?
> 
> There's no encoding for CMOVcc which takes an $imm.

Ah.

> %r10 is guaranteed zero after the FineIBT prologue

If the "jz" in the FineIBT prologue mispredicts, isn't %r10 non-zero by
definition?

> , but I don't see
> anything in patch 11 which makes this true in the !FineIBT case.

I thought this code is only used by FineIBT?

-- 
Josh