x86/lib: Add fast-short-rep-movs check to copy_user_enhanced_fast_string()

Commit

  f444a5ff95 ("x86/cpufeatures: Add support for fast short REP; MOVSB")

fixed memmove() with an ALTERNATIVE that will use REP MOVSB for all
string lengths.

copy_user_enhanced_fast_string() has a similar run time check to avoid
using REP MOVSB for copies less that 64 bytes.

Add an ALTERNATIVE to patch out the short length check and always use
REP MOVSB on X86_FEATURE_FSRM CPUs.

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20211216172431.1396371-1-tony.luck@intel.com

arch/x86/lib/copy_user_64.S

@@ -200,8 +200,8 @@ EXPORT_SYMBOL(copy_user_generic_string)
  */
 SYM_FUNC_START(copy_user_enhanced_fast_string)
 	ASM_STAC
-	cmpl $64,%edx
-	jb .L_copy_short_string	/* less then 64 bytes, avoid the costly 'rep' */
+	/* CPUs without FSRM should avoid rep movsb for short copies */
+	ALTERNATIVE "cmpl $64, %edx; jb .L_copy_short_string", "", X86_FEATURE_FSRM
 	movl %edx,%ecx
 1:	rep
 	movsb