x86/kexec: Use correct swap page in swap_pages function

The swap_pages function expects the swap page to be in %r10, but there
was no documentation to that effect. Once upon a time the setup code
used to load its value from a kernel virtual address and save it to an
address which is accessible in the identity-mapped page tables, and
*happened* to use %r10 to do so, with no comment that it was left there
on *purpose* instead of just being a scratch register. Once that was no
longer necessary, %r10 just holds whatever the kernel happened to leave
in it.

Now that the original value passed by the kernel is accessible via
%rip-relative addressing, load directly from there instead of using %r10
for it. But document the other parameters that the swap_pages function
*does* expect in registers.

Fixes: b3adabae8a ("x86/kexec: Drop page_list argument from relocate_kernel()")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20250109140757.2841269-4-dwmw2@infradead.org

arch/x86/kernel/relocate_kernel_64.S

@@ -264,6 +264,10 @@ SYM_CODE_END(virtual_mapped)
 	/* Do the copies */
 SYM_CODE_START_LOCAL_NOALIGN(swap_pages)
 	UNWIND_HINT_END_OF_STACK
+	/*
+	 * %rdi indirection page
+	 * %r11 preserve_context
+	 */
 	movq	%rdi, %rcx	/* Put the indirection_page in %rcx */
 	xorl	%edi, %edi
 	xorl	%esi, %esi
@@ -302,7 +306,7 @@ SYM_CODE_START_LOCAL_NOALIGN(swap_pages)
 	jz	.Lnoswap
 
 	/* copy source page to swap page */
-	movq	%r10, %rdi
+	movq	kexec_pa_swap_page(%rip), %rdi
 	movl	$512, %ecx
 	rep ; movsq
 
@@ -314,7 +318,7 @@ SYM_CODE_START_LOCAL_NOALIGN(swap_pages)
 
 	/* copy swap page to destination page */
 	movq	%rdx, %rdi
-	movq	%r10, %rsi
+	movq	kexec_pa_swap_page(%rip), %rsi
 .Lnoswap:
 	movl	$512, %ecx
 	rep ; movsq