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

From: Linus Torvalds
Date: Wed Apr 17 2019 - 12:26:46 EST

On Wed, Apr 17, 2019 at 1:50 AM Borislav Petkov <bp@xxxxxxxxx> 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

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

So I see no upside to changing it.