linux-kernel - [RFC PATCH v4 02/20] x86/kexec: Clean up and document register use in relocate_kernel

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Message-ID: <20241127190343.44916-3-dwmw2@infradead.org>
Date: Wed, 27 Nov 2024 19:00:16 +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>,
	Dave Young <dyoung@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	jpoimboe@...nel.org,
	bsz@...zon.de
Subject: [RFC PATCH v4 02/20] x86/kexec: Clean up and document register use in relocate_kernel_64.S

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

Add more comments explaining what each register contains, and save the
preserve_context flag to a non-clobbered register sooner, to keep things
simpler.

Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
Acked-by: Kai Huang <kai.huang@...el.com>
---
 arch/x86/kernel/relocate_kernel_64.S | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 1236f25fc8d1..92478e2e254f 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -100,6 +100,9 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
 	movq	%r10, CP_PA_SWAP_PAGE(%r11)
 	movq	%rdi, CP_PA_BACKUP_PAGES_MAP(%r11)
 
+	/* Save the preserve_context to %r11 as swap_pages clobbers %rcx. */
+	movq	%rcx, %r11
+
 	/* Switch to the identity mapped page tables */
 	movq	%r9, %cr3
 
@@ -116,6 +119,14 @@ SYM_CODE_END(relocate_kernel)
 
 SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	UNWIND_HINT_END_OF_STACK
+	/*
+	 * %rdi	indirection page
+	 * %rdx start address
+	 * %r11 preserve_context
+	 * %r12 host_mem_enc_active
+	 * %r13 original CR4 when relocate_kernel() was invoked
+	 */
+
 	/* set return address to 0 if not preserving context */
 	pushq	$0
 	/* store the start address on the stack */
@@ -170,8 +181,6 @@ 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
 
 	/*
@@ -183,13 +192,14 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	movq	%cr3, %rax
 	movq	%rax, %cr3
 
+	testq	%r11, %r11	/* preserve_context */
+	jnz .Lrelocate
+
 	/*
 	 * set all of the registers to known values
 	 * leave %rsp alone
 	 */
 
-	testq	%r11, %r11
-	jnz .Lrelocate
 	xorl	%eax, %eax
 	xorl	%ebx, %ebx
 	xorl    %ecx, %ecx
-- 
2.47.0