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

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Message-ID: <20250308013814.sa745d25m3ddlu2b@jpoimboe>
Date: Fri, 7 Mar 2025 17:38:14 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Ingo Molnar <mingo@...nel.org>, 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 Fri, Mar 07, 2025 at 03:29:00PM -0800, H. Peter Anvin wrote:
> 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.

We hopefully won't need those hacks much longer anyway, as I'll have
another series to propose removing frame pointers for x86-64.

x86-32 can keep frame pointers, but doesn't need the constraints.  It's
not supported for livepatch so it doesn't need to be 100% reliable.
Worst case, an unwind skips a frame, but the call address still shows up
on stack trace dumps prepended with '?'.

I plan to do the asm_call() series before the FP removal series because
it's presumably less disruptive, and it has a bunch more orthogonal
cleanups.

-- 
Josh