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

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Message-ID: <90B1074B-E7D4-4CE0-8A82-ADEB7BAED7AD@zytor.com>
Date: Fri, 07 Mar 2025 14:04:10 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: linux-kernel@...r.kernel.org,
        tip-bot2 for Josh Poimboeuf <tip-bot2@...utronix.de>,
        linux-tip-commits@...r.kernel.org
CC: Josh Poimboeuf <jpoimboe@...nel.org>, Ingo Molnar <mingo@...nel.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 4, 2025 2:36:24 AM PST, tip-bot2 for Josh Poimboeuf <tip-bot2@...utronix.de> wrote:
>The following commit has been merged into the x86/asm branch of tip:
>
>Commit-ID:     05844663b4fcf22bb3a1494615ae3f25852c9abc
>Gitweb:        https://git.kernel.org/tip/05844663b4fcf22bb3a1494615ae3f25852c9abc
>Author:        Josh Poimboeuf <jpoimboe@...nel.org>
>AuthorDate:    Sun, 02 Mar 2025 17:21:03 -08:00
>Committer:     Ingo Molnar <mingo@...nel.org>
>CommitterDate: Tue, 04 Mar 2025 11:21:40 +01:00
>
>x86/asm: Make ASM_CALL_CONSTRAINT conditional on frame pointers
>
>With frame pointers enabled, ASM_CALL_CONSTRAINT is used in an inline
>asm statement with a call instruction to force the compiler to set up
>the frame pointer before doing the call.
>
>Without frame pointers, no such constraint is needed.  Make it
>conditional on frame pointers.
>
>Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
>Signed-off-by: Ingo Molnar <mingo@...nel.org>
>Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>Cc: Linus Torvalds <torvalds@...ux-foundation.org>
>Cc: Brian Gerst <brgerst@...il.com>
>Cc: H. Peter Anvin <hpa@...or.com>
>Cc: linux-kernel@...r.kernel.org
>---
> arch/x86/include/asm/asm.h | 4 ++++
> 1 file changed, 4 insertions(+)
>
>diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
>index 0d268e6..f1db9e8 100644
>--- a/arch/x86/include/asm/asm.h
>+++ b/arch/x86/include/asm/asm.h
>@@ -232,7 +232,11 @@ register unsigned long current_stack_pointer asm(_ASM_SP);
>  * gets set up by the containing function.  If you forget to do this, objtool
>  * may print a "call without frame pointer save/setup" warning.
>  */
>+#ifdef CONFIG_UNWINDER_FRAME_POINTER
> #define ASM_CALL_CONSTRAINT "r" (__builtin_frame_address(0))
>+#else
>+#define ASM_CALL_CONSTRAINT
>+#endif
> 
> #endif /* __ASSEMBLY__ */
> 

So we are going to be using this version despite the gcc maintainers telling us it is not supported?