linux-kernel - [RFC PATCH 3/7] x86/kexec: Only swap pages for preserve

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Message-ID: <20241103054019.3795299-4-dwmw2@infradead.org>
Date: Sun,  3 Nov 2024 05:35:28 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: kexec@...ts.infradead.org
Cc: Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	David Woodhouse <dwmw@...zon.co.uk>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Kai Huang <kai.huang@...el.com>,
	Nikolay Borisov <nik.borisov@...e.com>,
	linux-kernel@...r.kernel.org,
	Simon Horman <horms@...nel.org>
Subject: [RFC PATCH 3/7] x86/kexec: Only swap pages for preserve_context mode

From: David Woodhouse <dwmw@...zon.co.uk>

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@...zon.co.uk>
---
 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 e607c0b7d70b..9b6ed08e00ea 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -302,6 +302,9 @@ 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 */
+	jnz .Lnoswap
+
 	/* copy source page to swap page */
 	movq	%r10, %rdi
 	movl	$512, %ecx
@@ -316,6 +319,7 @@ 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
 
-- 
2.44.0