x86/kexec: Add 8250 MMIO serial port output

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>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250326142404.256980-3-dwmw2@infradead.org

4 files changed, 43 insertions, 0 deletions

diff --git a/arch/x86/include/asm/kexec.h b/arch/x86/include/asm/kexec.h
index 9601094ac0ba..f2ad77929d6e 100644
--- a/arch/x86/include/asm/kexec.h
+++ b/arch/x86/include/asm/kexec.h
@@ -65,6 +65,7 @@ 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 acb7d05ab6af..9e8707d54f9c 100644
--- a/arch/x86/kernel/early_printk.c
+++ b/arch/x86/kernel/early_printk.c
@@ -333,6 +333,9 @@ 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);
+#if defined(CONFIG_KEXEC_CORE) && defined(CONFIG_X86_64)
+		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 cc73f9708464..ecb0da54abd1 100644
--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -76,6 +76,19 @@ 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
 
+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);
+}
+
 #ifdef CONFIG_KEXEC_FILE
 const struct kexec_file_ops * const kexec_file_loaders[] = {
 		&kexec_bzImage64_ops,
@@ -285,6 +298,10 @@ 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 21011cd19c48..8808cfca6322 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -39,6 +39,7 @@ 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)
 
 	.balign 16
@@ -413,6 +414,22 @@ 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).
@@ -423,6 +440,11 @@ 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