Re: [PATCH v2] x86/lib: Use REP MOVSB for large memcpy() on ERMS CPUs without FSRM

[Linus Torvalds]

On Tue, 10 Nov 2026 at 12:18, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>
> On November 10, 2026 9:53:07 AM PST, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >Let's not do that.
> >
> >Putting magic things like RET in alternatives is just a bad bad idea.
> >
> >               Linus
>
> Why is that, *given that it is in assembly*?

It's assembly, but it's assembly that potentially gets rewritten.
objtool already knows about __x86_return_thunk being special (although
I think it's the _target_, not the call site, it cares about).

I would like to not have magic in stuff that is in the alternatives
section. We've had issues with relocations etc before.

That said, grepping a bit for this, it turns out we already seem to be
doing that in memmove_64.S, although it was hidden from my first grep:

  #define CHECK_LEN     cmp $0x20, %rdx; jb 1f
  #define MEMMOVE_BYTES movq %rdx, %rcx; rep movsb; RET
  .Lmemmove_begin_forward:
        ALTERNATIVE_2 __stringify(CHECK_LEN), \
                      __stringify(CHECK_LEN; MEMMOVE_BYTES), X86_FEATURE_ERMS, \
                      __stringify(MEMMOVE_BYTES), X86_FEATURE_FSRM

which is pretty disgusting. But I guess that train has passed..

           Linus