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

[David Laight]

On Tue, 10 Nov 2026 12:17:51 -0800
"H. Peter Anvin" <hpa@xxxxxxxxx> wrote:

> On November 10, 2026 9:53:07 AM PST, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >On Wed, 4 Nov 2026 at 14:28, Usama Arif <usama.arif@xxxxxxxxx> wrote:  
> >>
> >> +       ALTERNATIVE __stringify(cmpq $0x20, %rcx; jb .Lhandle_tail), \
> >> +                   __stringify(rep movsb; RET), X86_FEATURE_FSRM  
> >
> >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*?
> 
> For C (or Rust, or C++, ...) it is obviously bad chicken.

Because RET is a horrid sequence subject to a lot of patching
to avoid multiple speculative execution issues.
If is were just the 0xc3 opcode byte it wouldn't be an issue.

David