amd64: clean up copyin/copyout

- move the PSL.AC comment to the fault handler
- stop testing for zero-sized ops. after several minutes of package
building there were no copyin calls with zero bytes and very few
copyout. the semantic of returning 0 in this case is preserved
- shorten exit paths by clearing %eax earlier
- replace xchg with 3 movs. this is what compilers do. a naive
benchmark on EPYC suggests about 1% increase in thoughput thanks to
this change.
- remove the useless movb %cl,%al from copyout. it looks like a
leftover from many years ago

Reviewed by:	kib
Approved by:	re (gjb)
Differential Revision:	https://reviews.freebsd.org/D17286

sys/amd64/amd64/support.S

@@ -404,10 +404,7 @@ END(fillw)
 .macro	COPYOUT smap erms
 	PUSH_FRAME_POINTER
 	movq	PCPU(CURPCB),%r9
-	/* Trap entry clears PSL.AC */
 	movq	$copy_fault,PCB_ONFAULT(%r9)
-	testq	%rdx,%rdx			/* anything to do? */
-	jz	2f
 
 	/*
 	 * Check explicitly for non-user addresses.  If 486 write protection
@@ -432,10 +429,20 @@ END(fillw)
 	cmpq	%rcx,%rax
 	ja	copy_fault
 
-	xchgq	%rdi,%rsi
-	/* bcopy(%rsi, %rdi, %rdx) */
+	/*
+	 * Set up arguments for rep movs*.
+	 */
+	movq	%rdi,%r8
+	movq	%rsi,%rdi
+	movq	%r8,%rsi
 	movq	%rdx,%rcx
 
+	/*
+	 * Set return value to zero. Remaining failure mode goes through
+	 * copy_fault.
+	 */
+	xorl	%eax,%eax
+
 	SMAP_DISABLE \smap
 .if	\erms == 0
 	cmpq	$15,%rcx
@@ -447,17 +454,16 @@ END(fillw)
 	andb	$7,%cl
 	jne	1f
 	SMAP_ENABLE \smap
-	xorl	%eax,%eax
 	movq	%rax,PCB_ONFAULT(%r9)
 	POP_FRAME_POINTER
 	ret
 .endif
+	ALIGN_TEXT
 1:
 	rep
 	movsb
+
 	SMAP_ENABLE \smap
-2:
-	xorl	%eax,%eax
 	movq	%rax,PCB_ONFAULT(%r9)
 	POP_FRAME_POINTER
 	ret
@@ -487,8 +493,6 @@ END(copyout_smap_erms)
 	PUSH_FRAME_POINTER
 	movq	PCPU(CURPCB),%r9
 	movq	$copy_fault,PCB_ONFAULT(%r9)
-	testq	%rdx,%rdx			/* anything to do? */
-	jz	2f
 
 	/*
 	 * make sure address is valid
@@ -500,9 +504,12 @@ END(copyout_smap_erms)
 	cmpq	%rcx,%rax
 	ja	copy_fault
 
-	xchgq	%rdi,%rsi
+	movq	%rdi,%r8
+	movq	%rsi,%rdi
+	movq	%r8,%rsi
 	movq	%rdx,%rcx
-	movb	%cl,%al
+
+	xorl	%eax,%eax
 
 	SMAP_DISABLE \smap
 .if \erms == 0
@@ -511,22 +518,20 @@ END(copyout_smap_erms)
 	shrq	$3,%rcx				/* copy longword-wise */
 	rep
 	movsq
-	movb	%al,%cl
+	movb	%dl,%cl
 	andb	$7,%cl				/* copy remaining bytes */
 	jne	1f
 	SMAP_ENABLE \smap
-	xorl	%eax,%eax
 	movq	%rax,PCB_ONFAULT(%r9)
 	POP_FRAME_POINTER
 	ret
 .endif
+	ALIGN_TEXT
 1:
 	rep
 	movsb
 
 	SMAP_ENABLE \smap
-2:
-	xorl	%eax,%eax
 	movq	%rax,PCB_ONFAULT(%r9)
 	POP_FRAME_POINTER
 	ret
@@ -549,9 +554,9 @@ ENTRY(copyin_smap_erms)
 END(copyin_smap_erms)
 
 	ALIGN_TEXT
+	/* Trap entry clears PSL.AC */
 copy_fault:
-	movq	PCPU(CURPCB),%rdx
-	movq	$0,PCB_ONFAULT(%rdx)
+	movq	$0,PCB_ONFAULT(%r9)
 	movl	$EFAULT,%eax
 	POP_FRAME_POINTER
 	ret