linux-kernel - Re: [PATCH 22/23] x86/nospec: Refactor UNTRAIN_RET[

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Message-ID: <20230826154205.6ejzfk6mmrrtvsm5@treble>
Date:   Sat, 26 Aug 2023 08:42:05 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Nikolay Borisov <nik.borisov@...e.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Babu Moger <babu.moger@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>, David.Kaplan@....com,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        gregkh@...uxfoundation.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 22/23] x86/nospec: Refactor UNTRAIN_RET[_*]

On Fri, Aug 25, 2023 at 09:22:10PM +0300, Nikolay Borisov wrote:
> 
> 
> On 25.08.23 г. 10:01 ч., Josh Poimboeuf wrote:
> > Factor out the UNTRAIN_RET[_*] common bits into a helper macro.
> > 
> > Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> > ---
> >   arch/x86/include/asm/nospec-branch.h | 31 +++++++++-------------------
> >   1 file changed, 10 insertions(+), 21 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> > index 51e3f1a287d2..dcc78477a38d 100644
> > --- a/arch/x86/include/asm/nospec-branch.h
> > +++ b/arch/x86/include/asm/nospec-branch.h
> > @@ -288,35 +288,24 @@
> >    * As such, this must be placed after every *SWITCH_TO_KERNEL_CR3 at a point
> >    * where we have a stack but before any RET instruction.
> >    */
> > -.macro UNTRAIN_RET
> > +.macro __UNTRAIN_RET ibpb_feature, call_depth_insns
> >   #if defined(CONFIG_RETHUNK) || defined(CONFIG_CPU_IBPB_ENTRY)
> >   	VALIDATE_UNRET_END
> >   	ALTERNATIVE_3 "",						\
> >   		      CALL_UNTRAIN_RET, X86_FEATURE_UNRET,		\
> > -		      "call entry_ibpb", X86_FEATURE_ENTRY_IBPB,	\
> > -		     __stringify(RESET_CALL_DEPTH), X86_FEATURE_CALL_DEPTH
> > +		      "call entry_ibpb", \ibpb_feature,			\
> > +		     __stringify(\call_depth_insns), X86_FEATURE_CALL_DEPTH
> 
> so this becomes __stringify(__stringify(RESET_CALL_DEPTH)) etc.

Apparently the gas macro un-stringifies the argument when using it, so
it needs to be re-stringified again.  ¯\_(ツ)_/¯

-- 
Josh