x86: don't use REP_GOOD or ERMS for user memory copies

The modern target to use is FSRM (Fast Short REP MOVS), and the other
cases should only be used for bigger areas (ie mainly things like page
clearing).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: torvalds

arch/x86/include/asm/uaccess_64.h

@@ -18,9 +18,7 @@
 
 /* Handles exceptions in both to and from, but doesn't do access_ok */
 __must_check unsigned long
-copy_user_enhanced_fast_string(void *to, const void *from, unsigned len);
-__must_check unsigned long
-copy_user_generic_string(void *to, const void *from, unsigned len);
+copy_user_fast_string(void *to, const void *from, unsigned len);
 __must_check unsigned long
 copy_user_generic_unrolled(void *to, const void *from, unsigned len);
 
@@ -30,15 +28,12 @@ copy_user_generic(void *to, const void *from, unsigned len)
 	unsigned ret;
 
 	/*
-	 * If CPU has ERMS feature, use copy_user_enhanced_fast_string.
-	 * Otherwise, if CPU has rep_good feature, use copy_user_generic_string.
+	 * If CPU has FSRM feature, use 'rep movs'.
 	 * Otherwise, use copy_user_generic_unrolled.
 	 */
-	alternative_call_2(copy_user_generic_unrolled,
-			 copy_user_generic_string,
-			 X86_FEATURE_REP_GOOD,
-			 copy_user_enhanced_fast_string,
-			 X86_FEATURE_ERMS,
+	alternative_call(copy_user_generic_unrolled,
+			 copy_user_fast_string,
+			 X86_FEATURE_FSRM,
 			 ASM_OUTPUT2("=a" (ret), "=D" (to), "=S" (from),
 				     "=d" (len)),
 			 "1" (to), "2" (from), "3" (len)

arch/x86/lib/copy_user_64.S

@@ -104,8 +104,8 @@ SYM_FUNC_START(copy_user_generic_unrolled)
 SYM_FUNC_END(copy_user_generic_unrolled)
 EXPORT_SYMBOL(copy_user_generic_unrolled)
 
-/* Some CPUs run faster using the string copy instructions.
- * This is also a lot simpler. Use them when possible.
+/*
+ * Some CPUs support FSRM for Fast Short REP MOVS.
  *
  * Only 4GB of copy is supported. This shouldn't be a problem
  * because the kernel normally only writes from/to page sized chunks
@@ -122,58 +122,21 @@ EXPORT_SYMBOL(copy_user_generic_unrolled)
  * Output:
  * eax uncopied bytes or 0 if successful.
  */
-SYM_FUNC_START(copy_user_generic_string)
+SYM_FUNC_START(copy_user_fast_string)
 	ASM_STAC
-	cmpl $8,%edx
-	jb 2f		/* less than 8 bytes, go to byte copy loop */
-	ALIGN_DESTINATION
 	movl %edx,%ecx
-	shrl $3,%ecx
-	andl $7,%edx
-1:	rep movsq
-2:	movl %edx,%ecx
-3:	rep movsb
+1:	rep movsb
 	xorl %eax,%eax
 	ASM_CLAC
 	RET
 
-11:	leal (%rdx,%rcx,8),%ecx
-12:	movl %ecx,%edx		/* ecx is zerorest also */
-	jmp .Lcopy_user_handle_tail
-
-	_ASM_EXTABLE_CPY(1b, 11b)
-	_ASM_EXTABLE_CPY(3b, 12b)
-SYM_FUNC_END(copy_user_generic_string)
-EXPORT_SYMBOL(copy_user_generic_string)
-
-/*
- * Some CPUs are adding enhanced REP MOVSB/STOSB instructions.
- * It's recommended to use enhanced REP MOVSB/STOSB if it's enabled.
- *
- * Input:
- * rdi destination
- * rsi source
- * rdx count
- *
- * Output:
- * eax uncopied bytes or 0 if successful.
- */
-SYM_FUNC_START(copy_user_enhanced_fast_string)
-	ASM_STAC
-	/* CPUs without FSRM should avoid rep movsb for short copies */
-	ALTERNATIVE "cmpl $64, %edx; jb copy_user_short_string", "", X86_FEATURE_FSRM
-	movl %edx,%ecx
-1:	rep movsb
-	xorl %eax,%eax
+12:	movl %ecx,%eax		/* ecx is zerorest also */
 	ASM_CLAC
 	RET
 
-12:	movl %ecx,%edx		/* ecx is zerorest also */
-	jmp .Lcopy_user_handle_tail
-
 	_ASM_EXTABLE_CPY(1b, 12b)
-SYM_FUNC_END(copy_user_enhanced_fast_string)
-EXPORT_SYMBOL(copy_user_enhanced_fast_string)
+SYM_FUNC_END(copy_user_fast_string)
+EXPORT_SYMBOL(copy_user_fast_string)
 
 /*
  * Try to copy last bytes and clear the rest if needed.