linux-kernel - Re: [PATCH] powerpc/irq: Modernise inline assembly in irq_soft_mask

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Date:   Fri, 23 Sep 2022 17:15:43 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/irq: Modernise inline assembly in irq_soft_mask_{set,return}

On Sat, Sep 24, 2022 at 02:26:52AM +1000, Nicholas Piggin wrote:
> I still don't see what clauses guarantees asm("%0" ::"r"(foo)) to give
> 13. It doesn't say access via inline assembly is special,

But it is.  It is for all register variables, local and global.  I agree
this isn't documented clearly.  For local register variables this is the
*only* thing guaranteed; for global register vars there is more (it
changes the ABI, there are safe/restore effects, that kind of thing).

Never it is guaranteed that all accesses through this variable will use
the register directly: this fundamentally cannot work on all archs, and
also not at -O0.  More in general it doesn't work if some basic
optimisations are not done, be it because of a compiler deficiency, or a
straight out bug, or maybe it is a conscious choice in some cases.

> I think if it was obviously guaranteed then this might be marginally
> better than explicit r13 in the asm
> 
>        asm volatile(
>                "stb %0,%2(%1)"
>                :
>                : "r" (mask),
> 	         "r" (local_paca),
>                  "i" (offsetof(struct paca_struct, irq_soft_mask))
>                : "memory");

(Please use "n" instead of "i".  Doesn't matter here, but it does in
many other places.)

Segher