Re: [PATCH] x86: add back the alignment of the destination to 8 bytes in copy_user_generic()

[David Laight <david.laight.linux@gmail.com>]

On Fri, 14 Mar 2025 09:06:13 -1000
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Fri, 14 Mar 2025 at 07:53, Herton R. Krzesinski <herton@redhat.com> wrote:
> >
> > --- a/arch/x86/include/asm/uaccess_64.h
> > +++ b/arch/x86/include/asm/uaccess_64.h
> > @@ -130,7 +130,7 @@ copy_user_generic(void *to, const void *from, unsigned long len)
> >                 "2:\n"
> >                 _ASM_EXTABLE_UA(1b, 2b)
> >                 :"+c" (len), "+D" (to), "+S" (from), ASM_CALL_CONSTRAINT
> > -               : : "memory", "rax");
> > +               : : "memory", "rax", "rdx", "r8");  
> 
> Please don't penalize the caller with the extra clobbers.
> 
> Maybe it doesn't matter - these functions are marked always_inline,
> but they aren't inlined in very many places and maybe those places
> have registers to spare - but let's not penalize the FSRM case anyway.
> 
> And we do call it "rep_movs_alternative", so let's keep it close to
> "rep movs" semantics (yes, we already clobber %rax, but let's not make
> it worse).
> 
> As to the actual change to rep_movs - that should be done differently
> too. In particular, I doubt it makes any sense to try to align the
> destination for small writes or for the ERMS case when we use 'rep
> movsb', so I think this should all go into just the ".Llarge_movsq"
> case.

The Intel cpu (sandy bridge onwards) execute 'rep mosvb' twice as
fast if the destination is 32 byte aligned.
Potentially this is worth optimising for - but the cost of the extra
code may exceed the benefit.

> .. and then the patch can be further optimized to just do the first -
> possibly unaligned - destination word unconditionally, and then
> updating the addresses and counts to make the rest be aligned.

You can also something similar for any trailing bytes.
If you are feeling 'brave' copy the last 8 bytes first.

	David