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

[Kiryl Shutsemau]

On Tue, Oct 13, 2026 at 03:04:42AM -0700, Usama Arif wrote:
>  SYM_TYPED_FUNC_START(__memcpy)
> -	ALTERNATIVE "jmp memcpy_orig", "", X86_FEATURE_FSRM
> +	ALTERNATIVE_2 "jmp memcpy_orig", \
> +		      "cmpq $512, %rdx; jb memcpy_orig", X86_FEATURE_ERMS, \
> +		      "", X86_FEATURE_FSRM

This grows the patch site from 2 bytes to 13. On FSRM the replacement is
empty, and 13 bytes is more than ASM_NOP_MAX, so add_nop() puts an
unconditional jump here, executed on every memcpy() call on FSRM
machines. :/

Maybe it is worth moving the patch site into memcpy_orig()? It already
has a 32-byte check. Which brings me to the other question:

Why 512? memmove() uses REP MOVSB from 32 bytes on ERMS. Can we do the
same here? Then we don't need a new branch at all.

--- a/arch/x86/lib/memcpy_64.S
+++ b/arch/x86/lib/memcpy_64.S
@@ -50,6 +50,8 @@
 	cmpq $0x20, %rdx
 	jb .Lhandle_tail
 
+	ALTERNATIVE "", __stringify(movq %rdx, %rcx; rep movsb; RET), X86_FEATURE_ERMS
+
 	/*
 	 * We check whether memory false dependence could occur,
 	 * then jump to corresponding copy mode.
-- 
  Kiryl Shutsemau / Kirill A. Shutemov