x86/kexec: Add comments around swap_pages() assembly to improve readability

The current assembly around swap_pages() in the relocate_kernel() takes
some time to follow because the use of registers can be easily lost when
the line of assembly goes long.  Add a couple of comments to clarify the
code around swap_pages() to improve readability.
Signed-off-by: Kai Huang <kai.huang@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Link: https://lore.kernel.org/all/8b52b0b8513a34b2a02fb4abb05c6700c2821475.1724573384.git.kai.huang@intel.com

arch/x86/kernel/relocate_kernel_64.S

@@ -170,6 +170,7 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	wbinvd
 .Lsme_off:
 
+	/* Save the preserve_context to %r11 as swap_pages clobbers %rcx. */
 	movq	%rcx, %r11
 	call	swap_pages
 
@@ -289,18 +290,21 @@ SYM_CODE_START_LOCAL_NOALIGN(swap_pages)
 	movq	%rcx,   %rsi  /* For ever source page do a copy */
 	andq	$0xfffffffffffff000, %rsi
 
-	movq	%rdi, %rdx
-	movq	%rsi, %rax
+	movq	%rdi, %rdx    /* Save destination page to %rdx */
+	movq	%rsi, %rax    /* Save source page to %rax */
 
+	/* copy source page to swap page */
 	movq	%r10, %rdi
 	movl	$512, %ecx
 	rep ; movsq
 
+	/* copy destination page to source page */
 	movq	%rax, %rdi
 	movq	%rdx, %rsi
 	movl	$512, %ecx
 	rep ; movsq
 
+	/* copy swap page to destination page */
 	movq	%rdx, %rdi
 	movq	%r10, %rsi
 	movl	$512, %ecx