[PATCH 2/2] x86/kexec: Add comments around swap_pages() assembly to improve readability
From: Kai Huang
Date: Mon Aug 19 2024 - 08:22:14 EST
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@xxxxxxxxx>
---
arch/x86/kernel/relocate_kernel_64.S | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index f7a3ca3dee53..e9e88c342f75 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/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
--
2.45.2