[PATCH v2 0/2] Misc comments fixup in relocate_kernel_64.S

[PATCH v2 0/2] Misc comments fixup in relocate_kernel_64.S

[Kai Huang]

The assembly code of relcocate_kernel() takes some time to follow for
newbies like me.  This series adds some comments to try to improve the
readability hoping they could be helpful to others too.  Also fix an
error (IIUC) in one comment.

v1 -> v2:
  - Add Kirill's tag.

  v1: https://lore.kernel.org/lkml/q2y5vte3wwn5qde5p4nfmjfqtzxfen3nhjdyafc7nbirfidpvr@ro3djjz3pub4/T/

Kai Huang (2):
  x86/kexec: Fix a comment of swap_pages() assembly
  x86/kexec: Add comments around swap_pages() assembly to improve
    readability

 arch/x86/kernel/relocate_kernel_64.S | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)


base-commit: e77f8f275278886d05ce6dfe9e3bc854e7bf0713
-- 
2.46.0

[PATCH v2 1/2] x86/kexec: Fix a comment of swap_pages() assembly

[Kai Huang]

When relocate_kernel() gets called, %rdi holds 'indirection_page' and
%rsi holds 'page_list'.  And %rdi always holds 'indirection_page' when
swap_pages() is called.

Therefore the comment of the first line code of swap_pages()

	movq    %rdi, %rcx      /* Put the page_list in %rcx */

.. isn't correct because it actually moves the 'indirection_page' to
the %rcx.  Fix it.

Signed-off-by: Kai Huang <kai.huang@intel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 arch/x86/kernel/relocate_kernel_64.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 042c9a0334e9..f7a3ca3dee53 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -258,7 +258,7 @@ SYM_CODE_END(virtual_mapped)
 	/* Do the copies */
 SYM_CODE_START_LOCAL_NOALIGN(swap_pages)
 	UNWIND_HINT_END_OF_STACK
-	movq	%rdi, %rcx	/* Put the page_list in %rcx */
+	movq	%rdi, %rcx	/* Put the indirection_page in %rcx */
 	xorl	%edi, %edi
 	xorl	%esi, %esi
 	jmp	1f
-- 
2.46.0

[PATCH v2 2/2] x86/kexec: Add comments around swap_pages() assembly to improve readability

[Kai Huang]

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>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 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
-- 
2.46.0

[tip: x86/cleanups] x86/kexec: Add comments around swap_pages() assembly to improve readability

[tip-bot2 for Kai Huang]

The following commit has been merged into the x86/cleanups branch of tip:

Commit-ID:     ea49cdb26e7cffc261ceb1db26707e6d337fa104
Gitweb:        https://git.kernel.org/tip/ea49cdb26e7cffc261ceb1db26707e6d337fa104
Author:        Kai Huang <kai.huang@intel.com>
AuthorDate:    Sun, 25 Aug 2024 20:18:06 +12:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Sun, 25 Aug 2024 14:29:39 +02:00

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 | 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 f7a3ca3..e9e88c3 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

[tip: x86/cleanups] x86/kexec: Fix a comment of swap_pages() assembly

[tip-bot2 for Kai Huang]

The following commit has been merged into the x86/cleanups branch of tip:

Commit-ID:     3c41ad39f179c0e41f585975eb6834cd14f286ec
Gitweb:        https://git.kernel.org/tip/3c41ad39f179c0e41f585975eb6834cd14f286ec
Author:        Kai Huang <kai.huang@intel.com>
AuthorDate:    Sun, 25 Aug 2024 20:18:05 +12:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Sun, 25 Aug 2024 14:29:39 +02:00

x86/kexec: Fix a comment of swap_pages() assembly

When relocate_kernel() gets called, %rdi holds 'indirection_page' and
%rsi holds 'page_list'.  And %rdi always holds 'indirection_page' when
swap_pages() is called.

Therefore the comment of the first line code of swap_pages()

	movq    %rdi, %rcx      /* Put the page_list in %rcx */

.. isn't correct because it actually moves the 'indirection_page' to
the %rcx.  Fix it.

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/adafdfb1421c88efce04420fc9a996c0e2ca1b34.1724573384.git.kai.huang@intel.com

---
 arch/x86/kernel/relocate_kernel_64.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 042c9a0..f7a3ca3 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -258,7 +258,7 @@ SYM_CODE_END(virtual_mapped)
 	/* Do the copies */
 SYM_CODE_START_LOCAL_NOALIGN(swap_pages)
 	UNWIND_HINT_END_OF_STACK
-	movq	%rdi, %rcx	/* Put the page_list in %rcx */
+	movq	%rdi, %rcx	/* Put the indirection_page in %rcx */
 	xorl	%edi, %edi
 	xorl	%esi, %esi
 	jmp	1f