[tip: x86/boot] x86/kexec: Only swap pages for ::preserve_context mode

[tip-bot2 for David Woodhouse]

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

Commit-ID:     9e5683e2d0b5584c51993908c5d0afa78e613492
Gitweb:        https://git.kernel.org/tip/9e5683e2d0b5584c51993908c5d0afa78e613492
Author:        David Woodhouse <dwmw@xxxxxxxxxxxx>
AuthorDate:    Thu, 05 Dec 2024 15:05:10 
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Fri, 06 Dec 2024 10:41:59 +01:00

x86/kexec: Only swap pages for ::preserve_context mode

There's no need to swap pages (which involves three memcopies for each
page) in the plain kexec case. Just do a single copy from source to
destination page.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Baoquan He <bhe@xxxxxxxxxx>
Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
Cc: Dave Young <dyoung@xxxxxxxxxx>
Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
Cc: Ard Biesheuvel <ardb@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Link: https://lore.kernel.org/r/20241205153343.3275139-5-dwmw2@xxxxxxxxxxxxx
---
 arch/x86/kernel/relocate_kernel_64.S | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index fea650f..ca7f1e1 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -310,6 +310,9 @@ SYM_CODE_START_LOCAL_NOALIGN(swap_pages)
 	movq	%rdi, %rdx    /* Save destination page to %rdx */
 	movq	%rsi, %rax    /* Save source page to %rax */
 
+	testq	%r11, %r11    /* Only actually swap for ::preserve_context */
+	jz	.Lnoswap
+
 	/* copy source page to swap page */
 	movq	%r10, %rdi
 	movl	$512, %ecx
@@ -324,6 +327,7 @@ SYM_CODE_START_LOCAL_NOALIGN(swap_pages)
 	/* copy swap page to destination page */
 	movq	%rdx, %rdi
 	movq	%r10, %rsi
+.Lnoswap:
 	movl	$512, %ecx
 	rep ; movsq