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

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  f444a5ff95dc ("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
This commit is contained in:
Tony Luck
2021-12-16 09:24:31 -08:00
committed by Borislav Petkov
parent 9c7e2634f6
commit 244122b4d2
1 changed files with 2 additions and 2 deletions
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4
arch/x86/lib/copy_user_64.S
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@ -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