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

[Josh Poimboeuf]

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@xxxxxxxxxx> wrote:
> >On Sat, Mar 08, 2025 at 12:05:29AM +0100, Ingo Molnar wrote:
> >> 
> >> * H. Peter Anvin <hpa@xxxxxxxxx> 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