linux-kernel - Re: [tip: x86/asm] x86/asm: Make ASM_CALL_CONSTRAINT conditional on frame pointers

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Message-ID: <A669251B-7414-4EE7-B0AD-735E845C0B5B@zytor.com>
Date: Fri, 07 Mar 2025 15:29:00 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>, Ingo Molnar <mingo@...nel.org>
CC: linux-kernel@...r.kernel.org,
        tip-bot2 for Josh Poimboeuf <tip-bot2@...utronix.de>,
        linux-tip-commits@...r.kernel.org,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Brian Gerst <brgerst@...il.com>, x86@...nel.org
Subject: Re: [tip: x86/asm] x86/asm: Make ASM_CALL_CONSTRAINT conditional on frame pointers

On March 7, 2025 3:21:57 PM PST, Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>On Sat, Mar 08, 2025 at 12:05:29AM +0100, Ingo Molnar wrote:
>> 
>> * H. Peter Anvin <hpa@...or.com> wrote:
>> 
>> > > #endif /* __ASSEMBLY__ */
>> > 
>> > So we are going to be using this version despite the gcc maintainers 
>> > telling us it is not supported?
>> 
>> No, neither patches are in the x86 tree at the moment.
>
>FWIW, the existing ASM_CALL_CONSTRAINT is also not supported, so this
>patch wouldn't have changed anything in that respect.
>
>Regardless I plan to post a new patch set soon with a bunch of cleanups.
>
>It will keep the existing ASM_CALL_CONSTRAINT in place for GCC, and will
>use the new __builtin_frame_address(0) input constraint for Clang only.
>
>There will be a new asm_call() interface to hide the mess.
>

Alternatively, you can co-opt the gcc BR I already filed on this and argue there that there are new reasons to support the alternate construct.