[PATCH v6 0/7] x86/kexec: Add exception handling for relocate_kernel

[PATCH v6 0/7] x86/kexec: Add exception handling for relocate_kernel

[David Woodhouse]

Debugging kexec failures is painful, as anything going wrong in execution
of the critical relocate_kernel() function tends to just lead to a triple
fault. Thus leading to *weeks* of my life that I won't get back. Having
hacked something up for my own use, I figured I should share it...

Add a CONFIG_KEXEC_DEBUG option which sets up a trivial exception
handler in that environment, and outputs to the early_printk serial
console if configured. Currently only 8250-compatible serial ports are
supported, but that could be extended.

I had to hack up QEMU support for a PCI serial port which matches what
the existing early_printk code can drive, and the *real* 8250_pci driver
doesn't seem to cope with that setup at all, but whatever... the kexec
code now drives the same 32-bit stride which is all that earlyprintk
supports. We can always add more later, if anyone cares.

Someone who cares might want to bring the i386 version into line with
this, although the lack of rip-based addressing makes all the PIC code a
bit harder.

David Woodhouse (7):
      x86/kexec: Add CONFIG_KEXEC_DEBUG option
      x86/kexec: Debugging support: load a GDT
      x86/kexec: Debugging support: Load an IDT and basic exception entry points
      x86/kexec: Debugging support: Dump registers on exception
      x86/kexec: Add 8250 serial port output
      x86/kexec: Add 8250 MMIO serial port output
      [DO NOT MERGE] x86/kexec: Add int3 in kexec path for testing

 arch/x86/Kconfig.debug               |   1 +
 arch/x86/include/asm/kexec.h         |   7 +
 arch/x86/kernel/early_printk.c       |   9 ++
 arch/x86/kernel/machine_kexec_64.c   |  45 +++++++
 arch/x86/kernel/relocate_kernel_64.S | 248 +++++++++++++++++++++++++++++++++++
 kernel/Kconfig.kexec                 |  11 ++
 6 files changed, 321 insertions(+)

v6:
 • Rebase onto already-merged fixes in tip/x86/boot.
 • Move CONFIG_KEXEC_DEBUG to generic kernel/Kconfig.kexec as Bartosz is
   working on an Arm64 version.

v5: https://lore.kernel.org/kexec/20241205153343.3275139-1-dwmw2@infradead.org/T/
 • Drop [RFC].
 • Drop _PAGE_NOPTISHADOW fix, which Ingo already took into tip/x86/urgent.
 • Add memory-mapped serial port support (32-bit MMIO spacing only).

v4 (RFC): https://lore.kernel.org/kexec/20241127190343.44916-1-dwmw2@infradead.org/T/
 • Add _PAGE_NOPTISHADOW fix for the identmap code.
 • Drop explicit map of control page, which was masking the identmap bug.

v3 (RFC): https://lore.kernel.org/kexec/20241125100815.2512-1-dwmw2@infradead.org/T/
 • Add CONFIG_KEXEC_DEBUG option and use earlyprintk config.
 • Allocate PGD separately from control page.
 • Explicitly map control page into identmap.

V2 (RFC): https://lore.kernel.org/kexec/20241122224715.171751-1-dwmw2@infradead.org/T/
 • Introduce linker script, start to clean up data access.

V1 (RFC): https://lore.kernel.org/kexec/20241103054019.3795299-1-dwmw2@infradead.org/T/
 • Initial proof-of-concept hacks.

[PATCH v6 1/7] x86/kexec: Add CONFIG_KEXEC_DEBUG option

[David Woodhouse]

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

This does nothing yet. Support for x86_64 will follow in subsequent
commits, and an Arm64 version is also being worked on.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 arch/x86/Kconfig.debug |  1 +
 kernel/Kconfig.kexec   | 11 +++++++++++
 2 files changed, 12 insertions(+)

diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
index 74777a97e394..c54cbd36c710 100644
--- a/arch/x86/Kconfig.debug
+++ b/arch/x86/Kconfig.debug
@@ -13,6 +13,7 @@ config X86_VERBOSE_BOOTUP
 
 config EARLY_PRINTK
 	bool "Early printk" if EXPERT
+	select HAVE_KEXEC_DEBUG if X86_64
 	default y
 	help
 	  Write kernel log output directly into the VGA buffer or to a serial
diff --git a/kernel/Kconfig.kexec b/kernel/Kconfig.kexec
index 4d111f871951..a65b49c083c9 100644
--- a/kernel/Kconfig.kexec
+++ b/kernel/Kconfig.kexec
@@ -17,6 +17,9 @@ config KEXEC_ELF
 config HAVE_IMA_KEXEC
 	bool
 
+config HAVE_KEXEC_DEBUG
+	bool
+
 config KEXEC
 	bool "Enable kexec system call"
 	depends on ARCH_SUPPORTS_KEXEC
@@ -95,6 +98,14 @@ config KEXEC_JUMP
 	  Jump between original kernel and kexeced kernel and invoke
 	  code in physical address mode via KEXEC
 
+config KEXEC_DEBUG
+	bool "Debug kexec transition"
+	depends on HAVE_KEXEC_DEBUG
+	help
+	  Faults during kexec can be difficult to debug. This installs exception
+	  handlers and attempts to report faults. On x86_64 this would use the
+	  serial port configured for earlyprintk.
+
 config CRASH_DUMP
 	bool "kernel crash dumps"
 	default ARCH_DEFAULT_CRASH_DUMP
-- 
2.47.0

[PATCH v6 2/7] x86/kexec: Debugging support: load a GDT

[David Woodhouse]

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

There are some failure modes which lead to triple-faults in the
relocate_kernel function, which is fairly much undebuggable for normal
mortals.

Adding a GDT in the relocate_kernel environment is step 1 towards being
able to catch faults and do something more useful.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 arch/x86/kernel/relocate_kernel_64.S | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index af2cd06ff318..c62f03808f18 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -39,6 +39,18 @@ SYM_DATA(kexec_pa_table_page, .quad 0)
 SYM_DATA(kexec_pa_swap_page, .quad 0)
 SYM_DATA_LOCAL(pa_backup_pages_map, .quad 0)
 
+#ifdef CONFIG_KEXEC_DEBUG
+	.balign 16
+SYM_DATA_START_LOCAL(kexec_debug_gdt)
+	.word   kexec_debug_gdt_end - kexec_debug_gdt - 1
+	.long   0
+	.word   0
+	.quad   0x00cf9a000000ffff      /* __KERNEL32_CS */
+	.quad   0x00af9a000000ffff      /* __KERNEL_CS */
+	.quad   0x00cf92000000ffff      /* __KERNEL_DS */
+SYM_DATA_END_LABEL(kexec_debug_gdt, SYM_L_LOCAL, kexec_debug_gdt_end)
+#endif /* CONFIG_KEXEC_DEBUG */
+
 	.section .text..relocate_kernel,"ax";
 	.code64
 SYM_CODE_START_NOALIGN(relocate_kernel)
@@ -115,6 +127,21 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	/* store the start address on the stack */
 	pushq   %rdx
 
+#ifdef CONFIG_KEXEC_DEBUG
+	/* Create a GDTR (16 bits limit, 64 bits addr) on stack */
+	leaq	kexec_debug_gdt(%rip), %rax
+	pushq	%rax
+	pushw	(%rax)
+
+	/* Load the GDT, put the stack back */
+	lgdt	(%rsp)
+	addq	$10, %rsp
+
+	/* Test that we can load segments */
+	movq	%ds, %rax
+	movq	%rax, %ds
+#endif /* CONFIG_KEXEC_DEBUG */
+
 	/*
 	 * Clear X86_CR4_CET (if it was set) such that we can clear CR0_WP
 	 * below.
-- 
2.47.0

[PATCH v6 3/7] x86/kexec: Debugging support: Load an IDT and basic exception entry points

[David Woodhouse]

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

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 arch/x86/include/asm/kexec.h         |  5 ++
 arch/x86/kernel/machine_kexec_64.c   | 23 ++++++++
 arch/x86/kernel/relocate_kernel_64.S | 82 ++++++++++++++++++++++++++++
 3 files changed, 110 insertions(+)

diff --git a/arch/x86/include/asm/kexec.h b/arch/x86/include/asm/kexec.h
index 8ad187462b68..ec7636f4f86a 100644
--- a/arch/x86/include/asm/kexec.h
+++ b/arch/x86/include/asm/kexec.h
@@ -8,6 +8,9 @@
 # define PA_PGD			2
 # define PA_SWAP_PAGE		3
 # define PAGES_NR		4
+#else
+/* Size of each exception handler referenced by the IDT */
+# define KEXEC_DEBUG_EXC_HANDLER_SIZE	6 /* pushi, pushi, 2-byte jmp */
 #endif
 
 # define KEXEC_CONTROL_PAGE_SIZE	4096
@@ -58,6 +61,8 @@ struct kimage;
 extern unsigned long kexec_va_control_page;
 extern unsigned long kexec_pa_table_page;
 extern unsigned long kexec_pa_swap_page;
+extern gate_desc kexec_debug_idt[];
+extern unsigned char kexec_debug_exc_vectors[];
 #endif
 
 /*
diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
index a68f5a0a9f37..535438375568 100644
--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -304,6 +304,26 @@ static void load_segments(void)
 		);
 }
 
+static void prepare_debug_idt(unsigned long control_page, unsigned long vec_ofs)
+{
+#ifdef CONFIG_KEXEC_DEBUG
+	gate_desc idtentry = { 0 };
+	int i;
+
+	idtentry.bits.p = 1;
+	idtentry.bits.type = GATE_TRAP;
+	idtentry.segment = __KERNEL_CS;
+	idtentry.offset_low = (control_page & 0xFFFF) + vec_ofs;
+	idtentry.offset_middle = (control_page >> 16) & 0xFFFF;
+	idtentry.offset_high = control_page >> 32;
+
+	for (i = 0; i < 16; i++) {
+		kexec_debug_idt[i] = idtentry;
+		idtentry.offset_low += KEXEC_DEBUG_EXC_HANDLER_SIZE;
+	}
+#endif
+}
+
 int machine_kexec_prepare(struct kimage *image)
 {
 	void *control_page = page_address(image->control_code_page);
@@ -321,6 +341,9 @@ int machine_kexec_prepare(struct kimage *image)
 	if (image->type == KEXEC_TYPE_DEFAULT)
 		kexec_pa_swap_page = page_to_pfn(image->swap_page) << PAGE_SHIFT;
 
+	prepare_debug_idt((unsigned long)__pa(control_page),
+			  (unsigned long)kexec_debug_exc_vectors - reloc_start);
+
 	__memcpy(control_page, __relocate_kernel_start, reloc_end - reloc_start);
 
 	set_memory_rox((unsigned long)control_page, 1);
diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index c62f03808f18..f5b134a81827 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -49,6 +49,12 @@ SYM_DATA_START_LOCAL(kexec_debug_gdt)
 	.quad   0x00af9a000000ffff      /* __KERNEL_CS */
 	.quad   0x00cf92000000ffff      /* __KERNEL_DS */
 SYM_DATA_END_LABEL(kexec_debug_gdt, SYM_L_LOCAL, kexec_debug_gdt_end)
+
+	.balign 8
+SYM_DATA_START(kexec_debug_idt)
+	.skip 0x100, 0x00
+SYM_DATA_END(kexec_debug_idt)
+
 #endif /* CONFIG_KEXEC_DEBUG */
 
 	.section .text..relocate_kernel,"ax";
@@ -113,6 +119,11 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
 	jmp	*%rsi
 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
 	/*
@@ -140,6 +151,15 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	/* Test that we can load segments */
 	movq	%ds, %rax
 	movq	%rax, %ds
+
+	/* Now an IDTR on the stack to load the IDT the kernel created */
+	leaq	kexec_debug_idt(%rip), %rsi
+	pushq	%rsi
+	pushw	$0xff
+	lidt	(%rsp)
+	addq	$10, %rsp
+
+	//int3
 #endif /* CONFIG_KEXEC_DEBUG */
 
 	/*
@@ -367,3 +387,65 @@ SYM_CODE_START_LOCAL_NOALIGN(swap_pages)
 	ret
 	int3
 SYM_CODE_END(swap_pages)
+
+#ifdef CONFIG_KEXEC_DEBUG
+SYM_CODE_START_NOALIGN(kexec_debug_exc_vectors)
+	/* Each of these is 6 bytes. */
+.macro vec_err exc
+	UNWIND_HINT_ENTRY
+	. = kexec_debug_exc_vectors + (\exc * KEXEC_DEBUG_EXC_HANDLER_SIZE)
+	nop
+	nop
+	pushq	$\exc
+	jmp	exc_handler
+.endm
+
+.macro vec_noerr exc
+	UNWIND_HINT_ENTRY
+	. = kexec_debug_exc_vectors + (\exc * KEXEC_DEBUG_EXC_HANDLER_SIZE)
+	pushq	$0
+	pushq	$\exc
+	jmp	exc_handler
+.endm
+
+	ANNOTATE_NOENDBR
+	vec_noerr 0 // #DE
+	vec_noerr 1 // #DB
+	vec_noerr 2 // #NMI
+	vec_noerr 3 // #BP
+	vec_noerr 4 // #OF
+	vec_noerr 5 // #BR
+	vec_noerr 6 // #UD
+	vec_noerr 7 // #NM
+	vec_err 8   // #DF
+	vec_noerr 9
+	vec_err 10 // #TS
+	vec_err 11 // #NP
+	vec_err 12 // #SS
+	vec_err 13 // #GP
+	vec_err 14 // #PF
+	vec_noerr 15
+SYM_CODE_END(kexec_debug_exc_vectors)
+
+SYM_CODE_START_LOCAL_NOALIGN(exc_handler)
+	pushq	%rax
+	pushq	%rdx
+	movw	$0x3f8, %dx
+	movb	$'A', %al
+	outb	%al, %dx
+	popq	%rdx
+	popq	%rax
+
+	/* Only return from int3 */
+	cmpq	$3, (%rsp)
+	jne	.Ldie
+
+	addq	$16, %rsp
+	iretq
+
+.Ldie:
+	hlt
+	jmp	.Ldie
+
+SYM_CODE_END(exc_handler)
+#endif /* CONFIG_KEXEC_DEBUG */
-- 
2.47.0

[PATCH v6 4/7] x86/kexec: Debugging support: Dump registers on exception

[David Woodhouse]

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

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

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 arch/x86/kernel/relocate_kernel_64.S | 106 +++++++++++++++++++++++++--
 1 file changed, 98 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index f5b134a81827..020f0f6e3e2e 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -119,11 +119,6 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
 	jmp	*%rsi
 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
 	/*
@@ -389,6 +384,69 @@ 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_NOENDBR
+	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
@@ -429,11 +487,43 @@ 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', ':', 0x28(%rsp)
+	print_reg 'E', 'r', 'r', ':', 0x30(%rsp)
+	print_reg 'r', 'i', 'p', ':', 0x38(%rsp)
+	print_reg 'r', 's', 'p', ':', 0x50(%rsp)
+
+	/* We spilled these to the stack */
+	print_reg 'r', 'a', 'x', ':', 0x20(%rsp)
+	print_reg 'r', 'b', 'x', ':', 0x18(%rsp)
+	print_reg 'r', 'c', 'x', ':', 0x10(%rsp)
+	print_reg 'r', 'd', 'x', ':', 0x08(%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

[PATCH v6 5/7] x86/kexec: Add 8250 serial port output

[David Woodhouse]

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

If a serial port was configured for early_printk, use it for debug output
from the relocate_kernel exception handler too.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 arch/x86/include/asm/kexec.h         |  1 +
 arch/x86/kernel/early_printk.c       |  6 +++++
 arch/x86/kernel/relocate_kernel_64.S | 39 +++++++++++++++++++++++-----
 3 files changed, 40 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/kexec.h b/arch/x86/include/asm/kexec.h
index ec7636f4f86a..8cbdb6fd10c2 100644
--- a/arch/x86/include/asm/kexec.h
+++ b/arch/x86/include/asm/kexec.h
@@ -63,6 +63,7 @@ extern unsigned long kexec_pa_table_page;
 extern unsigned long kexec_pa_swap_page;
 extern gate_desc kexec_debug_idt[];
 extern unsigned char kexec_debug_exc_vectors[];
+extern uint16_t kexec_debug_8250_port;
 #endif
 
 /*
diff --git a/arch/x86/kernel/early_printk.c b/arch/x86/kernel/early_printk.c
index 44f937015e1e..bf06866ee90a 100644
--- a/arch/x86/kernel/early_printk.c
+++ b/arch/x86/kernel/early_printk.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/console.h>
 #include <linux/kernel.h>
+#include <linux/kexec.h>
 #include <linux/init.h>
 #include <linux/string.h>
 #include <linux/screen_info.h>
@@ -141,6 +142,11 @@ static __init void early_serial_hw_init(unsigned divisor)
 	serial_out(early_serial_base, DLL, divisor & 0xff);
 	serial_out(early_serial_base, DLH, (divisor >> 8) & 0xff);
 	serial_out(early_serial_base, LCR, c & ~DLAB);
+
+#ifdef CONFIG_KEXEC_DEBUG
+	if (serial_in == io_serial_in)
+		kexec_debug_8250_port = early_serial_base;
+#endif
 }
 
 #define DEFAULT_BAUD 9600
diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 020f0f6e3e2e..9781a28c246a 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -38,6 +38,7 @@ SYM_DATA(kexec_va_control_page, .quad 0)
 SYM_DATA(kexec_pa_table_page, .quad 0)
 SYM_DATA(kexec_pa_swap_page, .quad 0)
 SYM_DATA_LOCAL(pa_backup_pages_map, .quad 0)
+SYM_DATA(kexec_debug_8250_port, .word 0)
 
 #ifdef CONFIG_KEXEC_DEBUG
 	.balign 16
@@ -385,24 +386,50 @@ SYM_CODE_END(swap_pages)
 
 #ifdef CONFIG_KEXEC_DEBUG
 /*
- * Generic 'print character' routine (as yet unimplemented)
+ * Generic 'print character' routine
  *  - %al: Character to be printed (may clobber %rax)
  *  - %rdx: MMIO address or port.
  */
-SYM_CODE_START_LOCAL_NOALIGN(pr_char)
+#define XMTRDY          0x20
+
+#define TXR             0       /*  Transmit register (WRITE) */
+#define LSR             5       /*  Line Status               */
+
+SYM_CODE_START_LOCAL_NOALIGN(pr_char_8250)
 	UNWIND_HINT_FUNC
 	ANNOTATE_NOENDBR
+	addw	$LSR, %dx
+	xchg	%al, %ah
+.Lxmtrdy_loop:
+	inb	%dx, %al
+	testb	$XMTRDY, %al
+	jnz	.Lready
+	rep nop
+	jmp .Lxmtrdy_loop
+
+.Lready:
+	subw	$LSR, %dx
+	xchg	%al, %ah
+	outb	%al, %dx
+pr_char_null:
+	ANNOTATE_NOENDBR
+
 	ANNOTATE_UNRET_SAFE
 	ret
-SYM_CODE_END(pr_char)
+SYM_CODE_END(pr_char_8250)
 
 /*
  * 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
+.macro pr_setup
+	leaq	pr_char_8250(%rip), %rsi
+	movw	kexec_debug_8250_port(%rip), %dx
+	testw	%dx, %dx
+	jnz	1f
+
+	leaq	pr_char_null(%rip), %rsi
+1:
 .endm
 
 /* Print the nybble in %bl, clobber %rax */
-- 
2.47.0

[PATCH v6 6/7] x86/kexec: Add 8250 MMIO serial port output

[David Woodhouse]

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

This supports the same 32-bit MMIO-mapped 8250 as the early_printk code.

It's not clear why the early_printk code supports this form and only this
form; the actual runtime 8250_pci doesn't seem to support it. But having
hacked up QEMU to expose such a device, early_printk does work with it,
and now so does the kexec debug code.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 arch/x86/include/asm/kexec.h         |  1 +
 arch/x86/kernel/early_printk.c       |  3 +++
 arch/x86/kernel/machine_kexec_64.c   | 22 ++++++++++++++++++++++
 arch/x86/kernel/relocate_kernel_64.S | 22 ++++++++++++++++++++++
 4 files changed, 48 insertions(+)

diff --git a/arch/x86/include/asm/kexec.h b/arch/x86/include/asm/kexec.h
index 8cbdb6fd10c2..5081d0b9e290 100644
--- a/arch/x86/include/asm/kexec.h
+++ b/arch/x86/include/asm/kexec.h
@@ -64,6 +64,7 @@ extern unsigned long kexec_pa_swap_page;
 extern gate_desc kexec_debug_idt[];
 extern unsigned char kexec_debug_exc_vectors[];
 extern uint16_t kexec_debug_8250_port;
+extern unsigned long kexec_debug_8250_mmio32;
 #endif
 
 /*
diff --git a/arch/x86/kernel/early_printk.c b/arch/x86/kernel/early_printk.c
index bf06866ee90a..14f548e095c9 100644
--- a/arch/x86/kernel/early_printk.c
+++ b/arch/x86/kernel/early_printk.c
@@ -296,6 +296,9 @@ static __init void early_pci_serial_init(char *s)
 		/* WARNING! assuming the address is always in the first 4G */
 		early_serial_base =
 			(unsigned long)early_ioremap(bar0 & PCI_BASE_ADDRESS_MEM_MASK, 0x10);
+#ifdef CONFIG_KEXEC_DEBUG
+		kexec_debug_8250_mmio32 = bar0 & PCI_BASE_ADDRESS_MEM_MASK;
+#endif
 		write_pci_config(bus, slot, func, PCI_COMMAND,
 				 cmdreg|PCI_COMMAND_MEMORY);
 	}
diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
index 535438375568..d47ca1a06a4d 100644
--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -76,6 +76,24 @@ map_acpi_tables(struct x86_mapping_info *info, pgd_t *level4p)
 static int map_acpi_tables(struct x86_mapping_info *info, pgd_t *level4p) { return 0; }
 #endif
 
+#ifdef CONFIG_KEXEC_DEBUG
+static int map_mmio_serial(struct x86_mapping_info *info, pgd_t *level4p)
+{
+	unsigned long mstart, mend;
+
+	if (!kexec_debug_8250_mmio32)
+		return 0;
+
+	mstart = kexec_debug_8250_mmio32 & PAGE_MASK;
+	mend = (kexec_debug_8250_mmio32 + PAGE_SIZE + 23) & PAGE_MASK;
+	pr_info("Map PCI serial at %lx - %lx\n", mstart, mend);
+	return kernel_ident_mapping_init(info, level4p, mstart, mend);
+}
+#else
+static int map_mmio_serial(struct x86_mapping_info *info, pgd_t *level4p) { return 0; }
+#endif
+
+
 #ifdef CONFIG_KEXEC_FILE
 const struct kexec_file_ops * const kexec_file_loaders[] = {
 		&kexec_bzImage64_ops,
@@ -285,6 +303,10 @@ static int init_pgtable(struct kimage *image, unsigned long control_page)
 	if (result)
 		return result;
 
+	result = map_mmio_serial(&info, image->arch.pgd);
+	if (result)
+		return result;
+
 	/*
 	 * This must be last because the intermediate page table pages it
 	 * allocates will not be control pages and may overlap the image.
diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 9781a28c246a..493b378713e5 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -38,6 +38,7 @@ SYM_DATA(kexec_va_control_page, .quad 0)
 SYM_DATA(kexec_pa_table_page, .quad 0)
 SYM_DATA(kexec_pa_swap_page, .quad 0)
 SYM_DATA_LOCAL(pa_backup_pages_map, .quad 0)
+SYM_DATA(kexec_debug_8250_mmio32, .quad 0)
 SYM_DATA(kexec_debug_8250_port, .word 0)
 
 #ifdef CONFIG_KEXEC_DEBUG
@@ -418,6 +419,22 @@ pr_char_null:
 	ret
 SYM_CODE_END(pr_char_8250)
 
+SYM_CODE_START_LOCAL_NOALIGN(pr_char_8250_mmio32)
+	UNWIND_HINT_FUNC
+	ANNOTATE_NOENDBR
+.Lxmtrdy_loop_mmio:
+	movb	(LSR*4)(%rdx), %ah
+	testb	$XMTRDY, %ah
+	jnz	.Lready_mmio
+	rep nop
+	jmp .Lxmtrdy_loop_mmio
+
+.Lready_mmio:
+	movb	%al, (%rdx)
+	ANNOTATE_UNRET_SAFE
+	ret
+SYM_CODE_END(pr_char_8250_mmio32)
+
 /*
  * Load pr_char function pointer into %rsi and load %rdx with whatever
  * that function wants to see there (typically port/MMIO address).
@@ -428,6 +445,11 @@ SYM_CODE_END(pr_char_8250)
 	testw	%dx, %dx
 	jnz	1f
 
+	leaq	pr_char_8250_mmio32(%rip), %rsi
+	movq	kexec_debug_8250_mmio32(%rip), %rdx
+	testq	%rdx, %rdx
+	jnz	1f
+
 	leaq	pr_char_null(%rip), %rsi
 1:
 .endm
-- 
2.47.0

[PATCH v6 7/7] [DO NOT MERGE] x86/kexec: Add int3 in kexec path for testing

[David Woodhouse]

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

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 arch/x86/kernel/relocate_kernel_64.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 493b378713e5..e4253102b234 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -156,7 +156,7 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	lidt	(%rsp)
 	addq	$10, %rsp
 
-	//int3
+	int3
 #endif /* CONFIG_KEXEC_DEBUG */
 
 	/*
-- 
2.47.0

Re: [PATCH v6 2/7] x86/kexec: Debugging support: load a GDT

[Ingo Molnar]

* David Woodhouse <dwmw2@infradead.org> wrote:

> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> There are some failure modes which lead to triple-faults in the
> relocate_kernel function, which is fairly much undebuggable for normal
> mortals.
> 
> Adding a GDT in the relocate_kernel environment is step 1 towards being
> able to catch faults and do something more useful.
> 
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> ---
>  arch/x86/kernel/relocate_kernel_64.S | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
> index af2cd06ff318..c62f03808f18 100644
> --- a/arch/x86/kernel/relocate_kernel_64.S
> +++ b/arch/x86/kernel/relocate_kernel_64.S
> @@ -39,6 +39,18 @@ SYM_DATA(kexec_pa_table_page, .quad 0)
>  SYM_DATA(kexec_pa_swap_page, .quad 0)
>  SYM_DATA_LOCAL(pa_backup_pages_map, .quad 0)
>  
> +#ifdef CONFIG_KEXEC_DEBUG
> +	.balign 16
> +SYM_DATA_START_LOCAL(kexec_debug_gdt)
> +	.word   kexec_debug_gdt_end - kexec_debug_gdt - 1
> +	.long   0
> +	.word   0
> +	.quad   0x00cf9a000000ffff      /* __KERNEL32_CS */
> +	.quad   0x00af9a000000ffff      /* __KERNEL_CS */
> +	.quad   0x00cf92000000ffff      /* __KERNEL_DS */
> +SYM_DATA_END_LABEL(kexec_debug_gdt, SYM_L_LOCAL, kexec_debug_gdt_end)
> +#endif /* CONFIG_KEXEC_DEBUG */

Yeah, so is there any reason (other than paranoia) why the early-early 
GDT and IDT shouldn't be unconditional? There's many ways for such an 
approach to bitrot, it's much better to not hide it behind a 
default-disabled debug option...

Some of the other bits, like the hard-coded serial debugging 
assumptions, probably need to be behind the debug option - but much of 
the new debug mechanism looks safe and generic and can be always-on, 
IMHO.

This would also throw regressions back into the face of whoever manages 
to introduce them, ideally. ;-)

Thanks,

	Ingo

Re: [PATCH v6 2/7] x86/kexec: Debugging support: load a GDT

[David Woodhouse]

On 23 February 2025 09:53:07 GMT, Ingo Molnar <mingo@kernel.org> wrote:
>
>* David Woodhouse <dwmw2@infradead.org> wrote:
>
>> From: David Woodhouse <dwmw@amazon.co.uk>
>> 
>> There are some failure modes which lead to triple-faults in the
>> relocate_kernel function, which is fairly much undebuggable for normal
>> mortals.
>> 
>> Adding a GDT in the relocate_kernel environment is step 1 towards being
>> able to catch faults and do something more useful.
>> 
>> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
>> ---
>>  arch/x86/kernel/relocate_kernel_64.S | 27 +++++++++++++++++++++++++++
>>  1 file changed, 27 insertions(+)
>> 
>> diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
>> index af2cd06ff318..c62f03808f18 100644
>> --- a/arch/x86/kernel/relocate_kernel_64.S
>> +++ b/arch/x86/kernel/relocate_kernel_64.S
>> @@ -39,6 +39,18 @@ SYM_DATA(kexec_pa_table_page, .quad 0)
>>  SYM_DATA(kexec_pa_swap_page, .quad 0)
>>  SYM_DATA_LOCAL(pa_backup_pages_map, .quad 0)
>>  
>> +#ifdef CONFIG_KEXEC_DEBUG
>> +	.balign 16
>> +SYM_DATA_START_LOCAL(kexec_debug_gdt)
>> +	.word   kexec_debug_gdt_end - kexec_debug_gdt - 1
>> +	.long   0
>> +	.word   0
>> +	.quad   0x00cf9a000000ffff      /* __KERNEL32_CS */
>> +	.quad   0x00af9a000000ffff      /* __KERNEL_CS */
>> +	.quad   0x00cf92000000ffff      /* __KERNEL_DS */
>> +SYM_DATA_END_LABEL(kexec_debug_gdt, SYM_L_LOCAL, kexec_debug_gdt_end)
>> +#endif /* CONFIG_KEXEC_DEBUG */
>
>Yeah, so is there any reason (other than paranoia) why the early-early 
>GDT and IDT shouldn't be unconditional? There's many ways for such an 
>approach to bitrot, it's much better to not hide it behind a 
>default-disabled debug option...
>
>Some of the other bits, like the hard-coded serial debugging 
>assumptions, probably need to be behind the debug option - but much of 
>the new debug mechanism looks safe and generic and can be always-on, 
>IMHO.
>
>This would also throw regressions back into the face of whoever manages 
>to introduce them, ideally. ;-)
>
>Thanks,
>
>	Ingo

Makes sense to me. I was just trying to be as unobtrusive as possible. In a test branch where I was trying to fix up the objtool vs. CFI pain, I did move the IDT/GDT setup entirely into the ASM code and remove the C code which clears them (before the call into relocate_kernel() which might now trap if we remove the __nocfi hack). I never did get objtool to tolerate both clang and GCC builds though.

I think even the serial output (tied as it is to earlyprintk setup) could reasonably be enabled by default too.