Re: [RFC PATCH] x86: prevent gcc from emitting rep movsq/stosq for inlined ops

[Linus Torvalds]

On Wed, 2 Apr 2025 at 11:17, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>
> Taking a leaf out of the repoline book, the ideal library call(s) would be:
>
>     CALL __x86_thunk_rep_{mov,stos}sb
>
> using the REP ABI (parameters in %rcx/%rdi/etc), rather than the SYSV ABI.

Yes. That's basically what 'rep_movs_alternative' does so that we can
basically do a

                ALTERNATIVE("rep movsb",
                            "call rep_movs_alternative",
ALT_NOT(X86_FEATURE_FSRM))

but we only do this for user space copies exactly because we don't
have a good way to do it for compiler-generated ones.

If gcc just did an out-of-line call, but used the 'rep movs' "calling
convention", we would be able to basically do the rewriting
dynamically, replacing the call with an inlined "rep movsb" where
appropriate.

                 Linus