x86/kexec: Debugging support: Dump registers on exception

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

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
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>
Link: https://lore.kernel.org/r/20250314173226.3062535-3-dwmw2@infradead.org

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 8f26ffd108490c5ef07e61c75446c2b11c5cba52..29cb399a34d456fbf47fa144050ec2f740637fdc 100644 (file)
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -379,6 +379,69 @@ SYM_CODE_START_LOCAL_NOALIGN(swap_pages)
        int3
 SYM_CODE_END(swap_pages)
 
+/*
+ * 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
@@ -422,17 +485,63 @@ SYM_CODE_START_LOCAL_NOALIGN(exc_handler)
        VALIDATE_UNRET_END
 
        pushq   %rax
+       pushq   %rbx
+       pushq   %rcx
        pushq   %rdx
-       movw    $0x3f8, %dx
-       movb    $'A', %al
-       outb    %al, %dx
-       popq    %rdx
-       popq    %rax
+       pushq   %rsi
+
+       /* Stack frame */
+#define EXC_SS         0x58 /* Architectural... */
+#define EXC_RSP                0x50
+#define EXC_EFLAGS     0x48
+#define EXC_CS         0x40
+#define EXC_RIP                0x38
+#define EXC_ERRORCODE  0x30 /* Either architectural or zero pushed by handler */
+#define EXC_EXCEPTION  0x28 /* Pushed by handler entry point */
+#define EXC_RAX                0x20 /* Pushed just above in exc_handler */
+#define EXC_RBX                0x18
+#define EXC_RCX                0x10
+#define EXC_RDX                0x08
+#define EXC_RSI                0x00
+
+       /* Set up %rdx/%rsi for debug output */
+       pr_setup
+
+       /* rip and exception info */
+       print_reg 'E', 'x', 'c', ':', EXC_EXCEPTION(%rsp)
+       print_reg 'E', 'r', 'r', ':', EXC_ERRORCODE(%rsp)
+       print_reg 'r', 'i', 'p', ':', EXC_RIP(%rsp)
+       print_reg 'r', 's', 'p', ':', EXC_RSP(%rsp)
+
+       /* We spilled these to the stack */
+       print_reg 'r', 'a', 'x', ':', EXC_RAX(%rsp)
+       print_reg 'r', 'b', 'x', ':', EXC_RBX(%rsp)
+       print_reg 'r', 'c', 'x', ':', EXC_RCX(%rsp)
+       print_reg 'r', 'd', 'x', ':', EXC_RDX(%rsp)
+       print_reg 'r', 's', 'i', ':', EXC_RSI(%rsp)
+
+       /* Other registers untouched */
+       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
 
        /* Only return from INT3 */
-       cmpq    $3, (%rsp)
+       cmpq    $3, EXC_EXCEPTION(%rsp)
        jne     .Ldie
 
+       popq    %rsi
+       popq    %rdx
+       popq    %rcx
+       popq    %rbx
+       popq    %rax
+
        addq    $16, %rsp
        iretq