x86/mm: Clean up register saving in the __enc_copy() assembly code

Clean up the use of PUSH and POP and when registers are saved in the
__enc_copy() assembly function in order to improve the readability of the code.

Move parameter register saving into general purpose registers earlier
in the code and move all the pushes to the beginning of the function
with corresponding pops at the end.

We do this to prepare fixes.

Tested-by: Gabriel Craciunescu <nix.or.die@gmail.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20180110192556.6026.74187.stgit@tlendack-t1.amdoffice.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/mm/mem_encrypt_boot.S

@@ -103,20 +103,19 @@ ENTRY(__enc_copy)
 	orq	$X86_CR4_PGE, %rdx
 	mov	%rdx, %cr4
 
+	push	%r15
+
+	movq	%rcx, %r9		/* Save kernel length */
+	movq	%rdi, %r10		/* Save encrypted kernel address */
+	movq	%rsi, %r11		/* Save decrypted kernel address */
+
 	/* Set the PAT register PA5 entry to write-protect */
-	push	%rcx
 	movl	$MSR_IA32_CR_PAT, %ecx
 	rdmsr
-	push	%rdx			/* Save original PAT value */
+	mov	%rdx, %r15		/* Save original PAT value */
 	andl	$0xffff00ff, %edx	/* Clear PA5 */
 	orl	$0x00000500, %edx	/* Set PA5 to WP */
 	wrmsr
-	pop	%rdx			/* RDX contains original PAT value */
-	pop	%rcx
-
-	movq	%rcx, %r9		/* Save kernel length */
-	movq	%rdi, %r10		/* Save encrypted kernel address */
-	movq	%rsi, %r11		/* Save decrypted kernel address */
 
 	wbinvd				/* Invalidate any cache entries */
 
@@ -138,12 +137,13 @@ ENTRY(__enc_copy)
 	jnz	1b			/* Kernel length not zero? */
 
 	/* Restore PAT register */
-	push	%rdx			/* Save original PAT value */
 	movl	$MSR_IA32_CR_PAT, %ecx
 	rdmsr
-	pop	%rdx			/* Restore original PAT value */
+	mov	%r15, %rdx		/* Restore original PAT value */
 	wrmsr
 
+	pop	%r15
+
 	ret
 .L__enc_copy_end:
 ENDPROC(__enc_copy)