[RFC PATCH v2 10/16] x86/kexec: Eliminate writes through kernel mapping of relocate_kernel page

[David Woodhouse]

From: David Woodhouse <dwmw at amazon.co.uk>

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 at amazon.co.uk>
---
 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 b9ad3ef0b982..5c6456467f08 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
 
-- 
2.47.0