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

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Date:   Tue, 09 Feb 2021 16:12:33 +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 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.

Thanks,

        tglx