Re: [PATCH] arm64/io: Remind compiler that there is a memory side effect

[Will Deacon <will@kernel.org>]

On Sun, Apr 03, 2022 at 09:47:47AM +0200, Ard Biesheuvel wrote:
> On Sun, 3 Apr 2022 at 09:47, Ard Biesheuvel <ardb@kernel.org> wrote:
> > On Sun, 3 Apr 2022 at 09:38, Andrew Pinski <pinskia@gmail.com> wrote:
> > > It might not be the most restricted fix but it is a fix.
> > > The best fix is to tell that you are writing to that location of memory.
> > > volatile asm does not do what you think it does.
> > > You didn't read further down about memory clobbers:
> > > https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Clobbers-and-Scratch-Registers
> > > Specifically this part:
> > > The "memory" clobber tells the compiler that the assembly code
> > > performs memory reads or writes to items other than those listed in
> > > the input and output operands
> > >
> >
> > So should we be using "m"(*addr) instead of "r"(addr) here?
> >
> > (along with the appropriately sized casts)
> 
> I mean "=m" not "m"

That can generate writeback addressing modes, which I think breaks
MMIO virtualisation. We usually end up using "Q" instead but the codegen
tends to be worse iirc.

In any case, for this specific problem I think we either need a fixed
compiler or some kbuild magic to avoid using it / disable the new behaviour.

We rely on 'asm volatile' not being elided in other places too.

Will