Re: [patch 05/12] x86/irq: Provide macro for inlining irq stack switching

Josh Poimboeuf
2021-02-09T16:24:06Z

On Tue, Feb 09, 2021 at 04:12:33PM +0100, Thomas Gleixner wrote:
> On Mon, Feb 08 2021 at 14:42, Josh Poimboeuf wrote:
> > On Thu, Feb 04, 2021 at 09:49:08PM +0100, Thomas Gleixner wrote:
> >> #ifdef CONFIG_X86_64
> >> +
> >> +#ifdef CONFIG_UNWINDER_FRAME_POINTER
> >> +# define IRQSTACK_CALL_CONSTRAINT , ASM_CALL_CONSTRAINT
> >> +#else
> >> +# define IRQSTACK_CALL_CONSTRAINT
> >> +#endif
> >
> > Is this really needed? i.e. does ASM_CALL_CONSTRAINT actually affect
> > code generation with !FRAME_POINTER?
>
> The problem is that if the asm inline is the first operation in a
> function some compilers insert the asm inline before setting up the
> frame pointer.
>
> That's actualy irrelevant here as the compiler cannot reorder against
> the C code leading to the asm inline. So we can probably replace it with
> a big fat comment.

Actually, I think keeping ASM_CALL_CONSTRAINT is a good idea.

What I meant was, is the #ifdef needed? My previous understanding was
that ASM_CALL_CONSTRAINT has no effect for !FRAME_POINTER (i.e., ORC).

So is there any reason to *not* have ASM_CALL_CONSTRAINT with ORC?

--
Josh