x86/kexec: Clean up register usage in relocate_kernel()

The memory encryption flag is passed in %r8 because that's where the
calling convention puts it. Instead of moving it to %r12 and then using
%r8 for other things, just leave it in %r8 and use other registers
instead.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/20241205153343.3275139-13-dwmw2@infradead.org

arch/x86/kernel/relocate_kernel_64.S

@@ -79,24 +79,18 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
 	movq	%cr4, %r13
 	movq	%r13, saved_cr4(%rip)
 
-	/* Save SME active flag */
-	movq	%r8, %r12
-
 	/* save indirection list for jumping back */
 	movq	%rdi, pa_backup_pages_map(%rip)
 
 	/* Save the preserve_context to %r11 as swap_pages clobbers %rcx. */
 	movq	%rcx, %r11
 
-	/* Physical address of control page */
-	movq    %rsi, %r8
-
 	/* setup a new stack at the end of the physical control page */
-	lea	PAGE_SIZE(%r8), %rsp
+	lea	PAGE_SIZE(%rsi), %rsp
 
 	/* jump to identity mapped page */
-	addq	$(identity_mapped - relocate_kernel), %r8
-	pushq	%r8
+	addq	$(identity_mapped - relocate_kernel), %rsi
+	pushq	%rsi
 	ANNOTATE_UNRET_SAFE
 	ret
 	int3
@@ -107,8 +101,9 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	/*
 	 * %rdi	indirection page
 	 * %rdx start address
+	 * %r8 host_mem_enc_active
+	 * %r9 page table page
 	 * %r11 preserve_context
-	 * %r12 host_mem_enc_active
 	 * %r13 original CR4 when relocate_kernel() was invoked
 	 */
 
@@ -161,7 +156,7 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	 * entries that will conflict with the now unencrypted memory
 	 * used by kexec. Flush the caches before copying the kernel.
 	 */
-	testq	%r12, %r12
+	testq	%r8, %r8
 	jz .Lsme_off
 	wbinvd
 .Lsme_off: