linux-kernel - Re: [RESEND][PATCH v3 06/17] static_call: Add basic static call infrastructure

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Date:   Thu, 26 Mar 2020 19:28:23 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <namit@...are.com>
Cc:     x86 <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "mhiramat@...nel.org" <mhiramat@...nel.org>,
        "bristot@...hat.com" <bristot@...hat.com>,
        "jbaron@...mai.com" <jbaron@...mai.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        tglx <tglx@...utronix.de>, "mingo@...nel.org" <mingo@...nel.org>,
        "hpa@...or.com" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>,
        "ard.biesheuvel@...aro.org" <ard.biesheuvel@...aro.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>
Subject: Re: [RESEND][PATCH v3 06/17] static_call: Add basic static call
 infrastructure

On Thu, Mar 26, 2020 at 06:09:07PM +0000, Nadav Amit wrote:

> I think that the kernel underutilizes the pure attribute in general.
> Building it with "-Wsuggest-attribute=pure” results in many warnings.
> Function pointers such kvm_x86_ops.get_XXX() could have been candidates to
> use the “pure” attribute.
> 
> The syntax is what you would expect:
> 
>   static void __attribute__((pure))(*ptr)(void);
> 

Well, I didn't in fact expect that, because an attribute is not a
type qualifier.

> However, you have a point, gcc does not appear to respect “pure” for
> function pointers and emits a warning it is ignored. GCC apparently only
> respects “const”. In contrast clang appears to respect the pure attribute
> for function pointers.

Still, we can probably make it happen for static_call(), since it is a
direct call to the trampoline, all we need to do is make sure the
trampoline is declared pure.

It does however mean that static_call() inherits all the dangers and
pit-falls of function pointers with some extra on top. It will be
impossible to validate this stuff.

That is, you can static_call_update() with a pointer to a !pure function
and you get to keep the pieces.