linux-kernel - [RFC PATCH v4 18/20] x86/kexec: Debugging support: Dump registers on exception

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Message-ID: <20241127190343.44916-19-dwmw2@infradead.org>
Date: Wed, 27 Nov 2024 19:00:32 +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 18/20] x86/kexec: Debugging support: Dump registers on exception

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

The actual serial output function is a no-op for now.

Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
---
 arch/x86/kernel/relocate_kernel_64.S | 104 ++++++++++++++++++++++++---
 1 file changed, 96 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 2179f691a45b..cd349b6d34a7 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -114,11 +114,6 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
 	int3
 SYM_CODE_END(relocate_kernel)
 
-#ifdef DEBUG
-	UNWIND_HINT_UNDEFINED
-	.balign 0x100	/* relocate_kernel will be overwritten with an IDT */
-#endif
-
 SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	UNWIND_HINT_END_OF_STACK
 	/*
@@ -368,6 +363,68 @@ SYM_CODE_START_LOCAL_NOALIGN(swap_pages)
 SYM_CODE_END(swap_pages)
 
 #ifdef CONFIG_KEXEC_DEBUG
+/*
+ * Generic 'print character' routine (as yet unimplemented)
+ *  - %al: Character to be printed (may clobber %rax)
+ *  - %rdx: MMIO address or port.
+ */
+SYM_CODE_START_LOCAL_NOALIGN(pr_char)
+	UNWIND_HINT_FUNC
+	ANNOTATE_UNRET_SAFE
+	ret
+SYM_CODE_END(pr_char)
+
+/*
+ * Load pr_char function pointer into %rsi and load %rdx with whatever
+ * that function wants to see there (typically port/MMIO address).
+ */
+.macro	pr_setup
+	/* No output; pr_char just returns */
+	leaq	pr_char(%rip), %rsi
+.endm
+
+/* Print the nybble in %bl, clobber %rax */
+SYM_CODE_START_LOCAL_NOALIGN(pr_nybble)
+	UNWIND_HINT_FUNC
+	movb	%bl, %al
+	nop
+	andb	$0x0f, %al
+	addb 	$0x30, %al
+	cmpb	$0x3a, %al
+	jb	1f
+	addb	$('a' - '0' - 10), %al
+	ANNOTATE_RETPOLINE_SAFE
+1:	jmp	*%rsi
+SYM_CODE_END(pr_nybble)
+
+SYM_CODE_START_LOCAL_NOALIGN(pr_qword)
+	UNWIND_HINT_FUNC
+	movq	$16, %rcx
+1:	rolq	$4, %rbx
+	call	pr_nybble
+	loop	1b
+	movb	$'\n', %al
+	ANNOTATE_RETPOLINE_SAFE
+	jmp	*%rsi
+SYM_CODE_END(pr_qword)
+
+.macro print_reg a, b, c, d, r
+	movb	$\a, %al
+	ANNOTATE_RETPOLINE_SAFE
+	call	*%rsi
+	movb	$\b, %al
+	ANNOTATE_RETPOLINE_SAFE
+	call	*%rsi
+	movb	$\c, %al
+	ANNOTATE_RETPOLINE_SAFE
+	call	*%rsi
+	movb	$\d, %al
+	ANNOTATE_RETPOLINE_SAFE
+	call	*%rsi
+	movq	\r, %rbx
+	call	pr_qword
+.endm
+
 SYM_CODE_START_NOALIGN(kexec_debug_exc_vectors)
 	/* Each of these is 6 bytes. */
 .macro vec_err exc
@@ -408,11 +465,42 @@ SYM_CODE_END(kexec_debug_exc_vectors)
 
 SYM_CODE_START_LOCAL_NOALIGN(exc_handler)
 	pushq	%rax
+	pushq	%rbx
+	pushq	%rcx
 	pushq	%rdx
-	movw	$0x3f8, %dx
-	movb	$'A', %al
-	outb	%al, %dx
+	pushq	%rsi
+
+	/* Set up %rdx/%rsi for debug output */
+	pr_setup
+
+	/* rip and exception info */
+	print_reg 'E', 'x', 'c', ':', 40(%rsp)
+	print_reg 'E', 'r', 'r', ':', 48(%rsp)
+	print_reg 'r', 'i', 'p', ':', 54(%rsp)
+
+	/* We spilled these to the stack */
+	print_reg 'r', 'a', 'x', ':', 32(%rsp)
+	print_reg 'r', 'b', 'x', ':', 24(%rsp)
+	print_reg 'r', 'c', 'x', ':', 16(%rsp)
+	print_reg 'r', 'd', 'x', ':', 8(%rsp)
+
+	/* Other registers */
+	print_reg 'r', 's', 'i', ':', (%rsp)
+	print_reg 'r', 'd', 'i', ':', %rdi
+	print_reg 'r', '8', ' ', ':', %r8
+	print_reg 'r', '9', ' ', ':', %r9
+	print_reg 'r', '1', '0', ':', %r10
+	print_reg 'r', '1', '1', ':', %r11
+	print_reg 'r', '1', '2', ':', %r12
+	print_reg 'r', '1', '3', ':', %r13
+	print_reg 'r', '1', '4', ':', %r14
+	print_reg 'r', '1', '5', ':', %r15
+	print_reg 'c', 'r', '2', ':', %cr2
+
+	popq	%rsi
 	popq	%rdx
+	popq	%rcx
+	popq	%rbx
 	popq	%rax
 
 	/* Only return from int3 */
-- 
2.47.0