Re: [PATCH v4 1/2] powerpc/uaccess: Implement unsafe_put_user() using 'asm goto'

Michael Ellerman
2020-05-06T01:35:48Z

Segher Boessenkool <segher@kernel.crashing.org> writes:
> On Tue, May 05, 2020 at 05:40:21PM +0200, Christophe Leroy wrote:
>> >>+#define __put_user_asm_goto(x, addr, label, op) \
>> >>+ asm volatile goto( \
>> >>+ "1: " op "%U1%X1 %0,%1 # put_user\n" \
>> >>+ EX_TABLE(1b, %l2) \
>> >>+ : \
>> >>+ : "r" (x), "m<>" (*addr) \
>> >
>> >The "m<>" here is breaking GCC 4.6.3, which we allegedly still support.
>> >
>> >Plain "m" works, how much does the "<>" affect code gen in practice?
>> >
>> >A quick diff here shows no difference from removing "<>".
>>
>> It was recommended by Segher, there has been some discussion about it on
>> v1 of this patch, see
>> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/4fdc2aba6f5e51887d1cd0fee94be0989eada2cd.1586942312.git.christophe.leroy@c-s.fr/
>>
>> As far as I understood that's mandatory on recent gcc to get the
>> pre-update form of the instruction. With older versions "m" was doing
>> the same, but not anymore.
>
> Yes. How much that matters depends on the asm. On older CPUs (6xx/7xx,
> say) the update form was just as fast as the non-update form. On newer
> or bigger CPUs it is usually executed just the same as an add followed
> by the memory access, so it just saves a bit of code size.

The update-forms are stdux, sthux etc. right?

I don't see any change in the number of those with or without the
constraint. That's using GCC 9.3.0.

>> Should we ifdef the "m<>" or "m" based on GCC
>> version ?
>
> That will be a lot of churn. Just make 4.8 minimum?

As I said in my other mail that's not really up to us. We could mandate
a higher minimum for powerpc, but I'd rather not.

I think for now I'm inclined to just drop the "<>", and we can revisit
in a release or two when hopefully GCC 4.8 has become the minimum.

cheers