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

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Message-ID: <87wnvhaz2q.fsf@nanos.tec.linutronix.de>
Date:   Tue, 09 Feb 2021 19:14:05 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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 10:22, Josh Poimboeuf wrote:
> 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?

You're right. No idea how I ended up with that ifdef.

Thanks,

        tglx