On Sun, Apr 03, 2022 at 09:47:47AM +0200, Ard Biesheuvel wrote: > On Sun, 3 Apr 2022 at 09:47, Ard Biesheuvel <a...@kernel.org> wrote: > > On Sun, 3 Apr 2022 at 09:38, Andrew Pinski <pins...@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
