linux-kernel - Re: [PATCH] asm/io: Correct output operand specification of the MMIO write* routines

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Date:   Wed, 17 Apr 2019 09:26:23 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Michael Matz <matz@...e.de>, x86-ml <x86@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] asm/io: Correct output operand specification of the MMIO
 write* routines

On Wed, Apr 17, 2019 at 1:50 AM Borislav Petkov <bp@...en8.de> wrote:
>
> I'm looking at
>
>   c1f64a58003f ("x86: MMIO and gcc re-ordering issue")
>
> and trying to figure out was there any particular reason the address to
> the MMIO write routines had to be an input operand?

It doesn't have to be an input operand, but as long as it's a "asm
volatile" it simply doesn't matter, and it won't be re-ordered or
optimized wrt other mmio accesses (that are also "asm volatile").

The memory clobber we have is to make sure that it's not re-ordered
with non-mmio accesses to other addresses (and thats' true for reads
_or_ writes, so both mmio read and mmio write have the memory
clobber).

So changing the input "m" to an output "+m" simply shouldn't matter.
There's no upside. You can't remove the memory clobber anyway, and you
can't remove the "asm volatile".

The "__" versions lack the memory clobber and aren't ordered wrt
normal memory (but are ordered wrt other mmio due to the "asm
volaile").

So I see no upside to changing it.

                  Linus