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

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Date:   Tue, 9 Feb 2021 10:22:14 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Kees Cook <keescook@...omium.org>
Subject: Re: [patch 05/12] x86/irq: Provide macro for inlining irq stack
 switching

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