x86: improve on the non-rep 'copy_user' function

The old 'copy_user_generic_unrolled' function was oddly implemented for
largely historical reasons: it had been largely based on the uncached
copy case, which has some other concerns.

For example, the __copy_user_nocache() function uses 'movnti' for the
destination stores, and those want the destination to be aligned. In
contrast, the regular copy function doesn't really care, and trying to
align things only complicates matters.

Also, like the clear_user function, the copy function had some odd
handling of the repeat counts, complicating the exception handling for
no really good reason. So as with clear_user, just write it to keep all
the byte counts in the %rcx register, exactly like the 'rep movs'
functionality that this replaces.

Unlike a real 'rep movs', we do allow for this to trash a few temporary
registers to not have to unnecessarily save/restore registers on the
stack.

And like the clearing case, rename this to what it now clearly is:
'rep_movs_alternative', and make it one coherent function, so that it
shows up as such in profiles (instead of the odd split between
"copy_user_generic_unrolled" and "copy_user_short_string", the latter of
which was not about strings at all, and which was shared with the
uncached case).

Signed-off-by: Linus Torvalds <[e-mail]>

arch/x86/include/asm/uaccess_64.h

···
 
 /* Handles exceptions in both to and from, but doesn't do access_ok */
 __must_check unsigned long
-copy_user_generic_unrolled(void *to, const void *from, unsigned len);
+rep_movs_alternative(void *to, const void *from, unsigned len);
 
 static __always_inline __must_check unsigned long
 copy_user_generic(void *to, const void *from, unsigned long len)
···
 	stac();
 	/*
 	 * If CPU has FSRM feature, use 'rep movs'.
-	 * Otherwise, use copy_user_generic_unrolled.
+	 * Otherwise, use rep_movs_alternative.
 	 */
 	asm volatile(
 		"1:\n\t"
 		ALTERNATIVE("rep movsb",
-			    "call copy_user_generic_unrolled", ALT_NOT(X86_FEATURE_FSRM))
+			    "call rep_movs_alternative", ALT_NOT(X86_FEATURE_FSRM))
 		"2:\n"
 		_ASM_EXTABLE_UA(1b, 2b)
 		:"+c" (len), "+D" (to), "+S" (from), ASM_CALL_CONSTRAINT
-		: : "memory", "rax", "rdx", "r8", "r9", "r10", "r11");
+		: : "memory", "rax", "r8", "r9", "r10", "r11");
 	clac();
 	return len;
 }

arch/x86/lib/copy_user_64.S

···
 #include <asm/export.h>
 #include <asm/trapnr.h>
 
+/*
+ * rep_movs_alternative - memory copy with exception handling.
+ * This version is for CPUs that don't have FSRM (Fast Short Rep Movs)
+ *
+ * Input:
+ * rdi destination
+ * rsi source
+ * rcx count
+ *
+ * Output:
+ * rcx uncopied bytes or 0 if successful.
+ *
+ * NOTE! The calling convention is very intentionally the same as
+ * for 'rep movs', so that we can rewrite the function call with
+ * just a plain 'rep movs' on machines that have FSRM.  But to make
+ * it simpler for us, we can clobber rsi/rdi and rax/r8-r11 freely.
+ */
+SYM_FUNC_START(rep_movs_alternative)
+	cmpq $64,%rcx
+	jae .Lunrolled
+
+	cmp $8,%ecx
+	jae .Lword
+
+	testl %ecx,%ecx
+	je .Lexit
+
+.Lcopy_user_tail:
+0:	movb (%rsi),%al
+1:	movb %al,(%rdi)
+	inc %rdi
+	inc %rsi
+	dec %rcx
+	jne .Lcopy_user_tail
+.Lexit:
+	RET
+
+	_ASM_EXTABLE_UA( 0b, .Lexit)
+	_ASM_EXTABLE_UA( 1b, .Lexit)
+
+	.p2align 4
+.Lword:
+2:	movq (%rsi),%rax
+3:	movq %rax,(%rdi)
+	addq $8,%rsi
+	addq $8,%rdi
+	sub $8,%ecx
+	je .Lexit
+	cmp $8,%ecx
+	jae .Lword
+	jmp .Lcopy_user_tail
+
+	_ASM_EXTABLE_UA( 2b, .Lcopy_user_tail)
+	_ASM_EXTABLE_UA( 3b, .Lcopy_user_tail)
+
+	.p2align 4
+.Lunrolled:
+10:	movq (%rsi),%r8
+11:	movq 8(%rsi),%r9
+12:	movq 16(%rsi),%r10
+13:	movq 24(%rsi),%r11
+14:	movq %r8,(%rdi)
+15:	movq %r9,8(%rdi)
+16:	movq %r10,16(%rdi)
+17:	movq %r11,24(%rdi)
+20:	movq 32(%rsi),%r8
+21:	movq 40(%rsi),%r9
+22:	movq 48(%rsi),%r10
+23:	movq 56(%rsi),%r11
+24:	movq %r8,32(%rdi)
+25:	movq %r9,40(%rdi)
+26:	movq %r10,48(%rdi)
+27:	movq %r11,56(%rdi)
+	addq $64,%rsi
+	addq $64,%rdi
+	subq $64,%rcx
+	cmpq $64,%rcx
+	jae .Lunrolled
+	cmpl $8,%ecx
+	jae .Lword
+	testl %ecx,%ecx
+	jne .Lcopy_user_tail
+	RET
+
+	_ASM_EXTABLE_UA(10b, .Lcopy_user_tail)
+	_ASM_EXTABLE_UA(11b, .Lcopy_user_tail)
+	_ASM_EXTABLE_UA(12b, .Lcopy_user_tail)
+	_ASM_EXTABLE_UA(13b, .Lcopy_user_tail)
+	_ASM_EXTABLE_UA(14b, .Lcopy_user_tail)
+	_ASM_EXTABLE_UA(15b, .Lcopy_user_tail)
+	_ASM_EXTABLE_UA(16b, .Lcopy_user_tail)
+	_ASM_EXTABLE_UA(17b, .Lcopy_user_tail)
+	_ASM_EXTABLE_UA(20b, .Lcopy_user_tail)
+	_ASM_EXTABLE_UA(21b, .Lcopy_user_tail)
+	_ASM_EXTABLE_UA(22b, .Lcopy_user_tail)
+	_ASM_EXTABLE_UA(23b, .Lcopy_user_tail)
+	_ASM_EXTABLE_UA(24b, .Lcopy_user_tail)
+	_ASM_EXTABLE_UA(25b, .Lcopy_user_tail)
+	_ASM_EXTABLE_UA(26b, .Lcopy_user_tail)
+	_ASM_EXTABLE_UA(27b, .Lcopy_user_tail)
+SYM_FUNC_END(rep_movs_alternative)
+EXPORT_SYMBOL(rep_movs_alternative)
+
+/*
+ * The uncached copy needs to align the destination for
+ * movnti and friends.
+ */
 .macro ALIGN_DESTINATION
 	/* check for bad alignment of destination */
 	movl %edi,%ecx
···
 	_ASM_EXTABLE_CPY(101b, .Lcopy_user_handle_align)
 .endm
 
-/*
- * copy_user_generic_unrolled - memory copy with exception handling.
- * This version is for CPUs like P4 that don't have efficient micro
- * code for rep movsq
- *
- * Input:
- * rdi destination
- * rsi source
- * rcx count
- *
- * Output:
- * rcx uncopied bytes or 0 if successful.
- *
- * NOTE! The calling convention is very intentionally the same as
- * for 'rep movs', so that we can rewrite the function call with
- * just a plain 'rep movs' on machines that have FSRM.
- *
- * HOWEVER! This function ends up having a lot of the code common
- * with __copy_user_nocache(), which is a normal C function, and
- * has a similar calling convention, but gets the 'count' in %rdx,
- * and returns the result in %rax.
- *
- * To share as much code as possible, we end up returning the
- * result in *both* %rcx/%rax, and we also move the initial count
- * into %rdx.
- *
- * We can clobber rdx/rsi/rdi and r8-r11
- */
-SYM_FUNC_START(copy_user_generic_unrolled)
-	movl %ecx,%edx
-	cmpl $8,%ecx
-	jb .Lcopy_user_short_string_bytes
-	ALIGN_DESTINATION
-	movl %edx,%ecx
-	andl $63,%edx
-	shrl $6,%ecx
-	jz copy_user_short_string
-1:	movq (%rsi),%r8
-2:	movq 1*8(%rsi),%r9
-3:	movq 2*8(%rsi),%r10
-4:	movq 3*8(%rsi),%r11
-5:	movq %r8,(%rdi)
-6:	movq %r9,1*8(%rdi)
-7:	movq %r10,2*8(%rdi)
-8:	movq %r11,3*8(%rdi)
-9:	movq 4*8(%rsi),%r8
-10:	movq 5*8(%rsi),%r9
-11:	movq 6*8(%rsi),%r10
-12:	movq 7*8(%rsi),%r11
-13:	movq %r8,4*8(%rdi)
-14:	movq %r9,5*8(%rdi)
-15:	movq %r10,6*8(%rdi)
-16:	movq %r11,7*8(%rdi)
-	leaq 64(%rsi),%rsi
-	leaq 64(%rdi),%rdi
-	decl %ecx
-	jnz 1b
-	jmp copy_user_short_string
-
-30:	shll $6,%ecx
-	addl %ecx,%edx
-	jmp .Lcopy_user_handle_tail
-
-	_ASM_EXTABLE_CPY(1b, 30b)
-	_ASM_EXTABLE_CPY(2b, 30b)
-	_ASM_EXTABLE_CPY(3b, 30b)
-	_ASM_EXTABLE_CPY(4b, 30b)
-	_ASM_EXTABLE_CPY(5b, 30b)
-	_ASM_EXTABLE_CPY(6b, 30b)
-	_ASM_EXTABLE_CPY(7b, 30b)
-	_ASM_EXTABLE_CPY(8b, 30b)
-	_ASM_EXTABLE_CPY(9b, 30b)
-	_ASM_EXTABLE_CPY(10b, 30b)
-	_ASM_EXTABLE_CPY(11b, 30b)
-	_ASM_EXTABLE_CPY(12b, 30b)
-	_ASM_EXTABLE_CPY(13b, 30b)
-	_ASM_EXTABLE_CPY(14b, 30b)
-	_ASM_EXTABLE_CPY(15b, 30b)
-	_ASM_EXTABLE_CPY(16b, 30b)
-SYM_FUNC_END(copy_user_generic_unrolled)
-EXPORT_SYMBOL(copy_user_generic_unrolled)
-
-/*
- * Try to copy last bytes and clear the rest if needed.
- * Since protection fault in copy_from/to_user is not a normal situation,
- * it is not necessary to optimize tail handling.
- * Don't try to copy the tail if machine check happened
- *
- * Input:
- * eax trap number written by ex_handler_copy()
- * rdi destination
- * rsi source
- * rdx count
- *
- * Output:
- * eax uncopied bytes or 0 if successful.
- */
-SYM_CODE_START_LOCAL(.Lcopy_user_handle_tail)
-	cmp $X86_TRAP_MC,%eax
-	je 3f
-
-	movl %edx,%ecx
-1:	rep movsb
-2:	mov %ecx,%eax
-	RET
-
-3:
-	movl %edx,%eax
-	movl %edx,%ecx
-	RET
-
-	_ASM_EXTABLE_CPY(1b, 2b)
-
-.Lcopy_user_handle_align:
-	addl %ecx,%edx			/* ecx is zerorest also */
-	jmp .Lcopy_user_handle_tail
-
-SYM_CODE_END(.Lcopy_user_handle_tail)
-
-/*
- * Finish memcpy of less than 64 bytes.  #AC should already be set.
- *
- * Input:
- * rdi destination
- * rsi source
- * rdx count (< 64)
- *
- * Output:
- * eax uncopied bytes or 0 if successful.
- */
-SYM_CODE_START_LOCAL(copy_user_short_string)
-	movl %edx,%ecx
-	andl $7,%edx
-	shrl $3,%ecx
-	jz .Lcopy_user_short_string_bytes
-18:	movq (%rsi),%r8
-19:	movq %r8,(%rdi)
-	leaq 8(%rsi),%rsi
-	leaq 8(%rdi),%rdi
-	decl %ecx
-	jnz 18b
-.Lcopy_user_short_string_bytes:
-	andl %edx,%edx
-	jz 23f
-	movl %edx,%ecx
-21:	movb (%rsi),%al
-22:	movb %al,(%rdi)
-	incq %rsi
-	incq %rdi
-	decl %ecx
-	jnz 21b
-23:	xor %eax,%eax
-	xor %ecx,%ecx
-	RET
-
-40:	leal (%rdx,%rcx,8),%edx
-	jmp 60f
-50:	movl %ecx,%edx		/* ecx is zerorest also */
-60:	jmp .Lcopy_user_handle_tail
-
-	_ASM_EXTABLE_CPY(18b, 40b)
-	_ASM_EXTABLE_CPY(19b, 40b)
-	_ASM_EXTABLE_CPY(21b, 50b)
-	_ASM_EXTABLE_CPY(22b, 50b)
-SYM_CODE_END(copy_user_short_string)
 
 /*
  * copy_user_nocache - Uncached memory copy with exception handling
···
 	_ASM_EXTABLE_CPY(31b, .L_fixup_4b_copy)
 	_ASM_EXTABLE_CPY(40b, .L_fixup_1b_copy)
 	_ASM_EXTABLE_CPY(41b, .L_fixup_1b_copy)
+
+/*
+ * Try to copy last bytes and clear the rest if needed.
+ * Since protection fault in copy_from/to_user is not a normal situation,
+ * it is not necessary to optimize tail handling.
+ * Don't try to copy the tail if machine check happened
+ *
+ * Input:
+ * eax trap number written by ex_handler_copy()
+ * rdi destination
+ * rsi source
+ * rdx count
+ *
+ * Output:
+ * eax uncopied bytes or 0 if successful.
+ */
+.Lcopy_user_handle_tail:
+	cmp $X86_TRAP_MC,%eax
+	je 3f
+
+	movl %edx,%ecx
+1:	rep movsb
+2:	mov %ecx,%eax
+	RET
+
+3:
+	movl %edx,%eax
+	RET
+
+	_ASM_EXTABLE_CPY(1b, 2b)
+
+.Lcopy_user_handle_align:
+	addl %ecx,%edx			/* ecx is zerorest also */
+	jmp .Lcopy_user_handle_tail
+
 SYM_FUNC_END(__copy_user_nocache)
 EXPORT_SYMBOL(__copy_user_nocache)

tools/objtool/check.c

···
 	"copy_mc_enhanced_fast_string",
 	"ftrace_likely_update", /* CONFIG_TRACE_BRANCH_PROFILING */
 	"rep_stos_alternative",
-	"copy_user_generic_unrolled",
+	"rep_movs_alternative",
 	"__copy_user_nocache",
 	NULL
 };