x86: don't clobber r8 nor use rcx.

There's really no reason to clobber r8 or pass the address in rcx.
We can safely use only two registers (which we already have to touch anyway)
to do the job.
Signed-off-by: NGlauber Costa <gcosta@redhat.com>
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/lib/getuser_64.S

@@ -36,10 +36,10 @@
 	.text
 ENTRY(__get_user_1)
 	CFI_STARTPROC
-	GET_THREAD_INFO(%r8)
-	cmpq threadinfo_addr_limit(%r8),%rcx
+	GET_THREAD_INFO(%rdx)
+	cmpq threadinfo_addr_limit(%rdx),%rax
 	jae bad_get_user
-1:	movzb (%rcx),%edx
+1:	movzb (%rax),%edx
 	xorl %eax,%eax
 	ret
 	CFI_ENDPROC
@@ -47,48 +47,48 @@ ENDPROC(__get_user_1)
 
 ENTRY(__get_user_2)
 	CFI_STARTPROC
-	GET_THREAD_INFO(%r8)
-	addq $1,%rcx
+	GET_THREAD_INFO(%rdx)
+	addq $1,%rax
 	jc 20f
-	cmpq threadinfo_addr_limit(%r8),%rcx
+	cmpq threadinfo_addr_limit(%rdx),%rax
 	jae 20f
-	decq   %rcx
-2:	movzwl (%rcx),%edx
+	decq   %rax
+2:	movzwl (%rax),%edx
 	xorl %eax,%eax
 	ret
-20:	decq    %rcx
+20:	decq    %rax
 	jmp	bad_get_user
 	CFI_ENDPROC
 ENDPROC(__get_user_2)
 
 ENTRY(__get_user_4)
 	CFI_STARTPROC
-	GET_THREAD_INFO(%r8)
-	addq $3,%rcx
+	GET_THREAD_INFO(%rdx)
+	addq $3,%rax
 	jc 30f
-	cmpq threadinfo_addr_limit(%r8),%rcx
+	cmpq threadinfo_addr_limit(%rdx),%rax
 	jae 30f
-	subq $3,%rcx
-3:	movl (%rcx),%edx
+	subq $3,%rax
+3:	movl (%rax),%edx
 	xorl %eax,%eax
 	ret
-30:	subq $3,%rcx
+30:	subq $3,%rax
 	jmp bad_get_user
 	CFI_ENDPROC
 ENDPROC(__get_user_4)
 
 ENTRY(__get_user_8)
 	CFI_STARTPROC
-	GET_THREAD_INFO(%r8)
-	addq $7,%rcx
+	GET_THREAD_INFO(%rdx)
+	addq $7,%rax
 	jc 40f
-	cmpq threadinfo_addr_limit(%r8),%rcx
+	cmpq threadinfo_addr_limit(%rdx),%rax
 	jae	40f
-	subq	$7,%rcx
-4:	movq (%rcx),%rdx
+	subq	$7,%rax
+4:	movq (%rax),%rdx
 	xorl %eax,%eax
 	ret
-40:	subq $7,%rcx
+40:	subq $7,%rax
 	jmp bad_get_user
 	CFI_ENDPROC
 ENDPROC(__get_user_8)

include/asm-x86/uaccess_64.h

@@ -90,8 +90,7 @@ extern int fixup_exception(struct pt_regs *regs);
 #define __get_user_x(size, ret, x, ptr)		      \
 	asm volatile("call __get_user_" #size	      \
 		     : "=a" (ret),"=d" (x)	      \
-		     : "c" (ptr)		      \
-		     : "r8")
+		     : "0" (ptr))		      \
 
 /* Careful: we have to cast the result to the type of the pointer
  * for sign reasons */