[tip: x86/boot] x86/kexec: Eliminate writes through kernel mapping of relocate_kernel page

[tip-bot2 for David Woodhouse]

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

Commit-ID:     b7155dfd4999211247cce40be2665c71235ab094
Gitweb:        https://git.kernel.org/tip/b7155dfd4999211247cce40be2665c71235ab094
Author:        David Woodhouse <dwmw@xxxxxxxxxxxx>
AuthorDate:    Thu, 05 Dec 2024 15:05:17 
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Fri, 06 Dec 2024 10:42:00 +01:00

x86/kexec: Eliminate writes through kernel mapping of relocate_kernel page

All writes to the relocate_kernel control page are now done *after* the
%cr3 switch via simple %rip-relative addressing, which means the DATA()
macro with its pointer arithmetic can also now be removed.

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/20241205153343.3275139-12-dwmw2@xxxxxxxxxxxxx
---
 arch/x86/kernel/relocate_kernel_64.S | 29 +++++++++++++--------------
 1 file changed, 14 insertions(+), 15 deletions(-)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index d52c3bb..739041c 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -61,21 +61,24 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
 	pushq %r15
 	pushf
 
-	movq	%rsp, saved_rsp(%rip)
-	movq	%cr0, %rax
-	movq	%rax, saved_cr0(%rip)
-	movq	%cr3, %rax
-	movq	%rax, saved_cr3(%rip)
-	movq	%cr4, %rax
-	movq	%rax, saved_cr4(%rip)
-
-	/* Save CR4. Required to enable the right paging mode later. */
-	movq	%rax, %r13
-
 	/* zero out flags, and disable interrupts */
 	pushq $0
 	popfq
 
+	/* Switch to the identity mapped page tables */
+	movq	%cr3, %rax
+	movq	kexec_pa_table_page(%rip), %r9
+	movq	%r9, %cr3
+
+	/* Save %rsp and CRs. */
+	movq    %rsp, saved_rsp(%rip)
+	movq	%rax, saved_cr3(%rip)
+	movq	%cr0, %rax
+	movq	%rax, saved_cr0(%rip)
+	/* Leave CR4 in %r13 to enable the right paging mode later. */
+	movq	%cr4, %r13
+	movq	%r13, saved_cr4(%rip)
+
 	/* Save SME active flag */
 	movq	%r8, %r12
 
@@ -85,10 +88,6 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
 	/* Save the preserve_context to %r11 as swap_pages clobbers %rcx. */
 	movq	%rcx, %r11
 
-	/* Switch to the identity mapped page tables */
-	movq	kexec_pa_table_page(%rip), %r9
-	movq	%r9, %cr3
-
 	/* Physical address of control page */
 	movq    %rsi, %r8