[PATCH v2] x86/boot: Add a message about ignored early NMIs

["NOMURA JUNICHI(野村 淳一)" <junichi.nomura@nec.com>]

Commit 78a509fba9c9 ("x86/boot: Ignore NMIs during very early boot") added
empty handler in early boot stage to avoid boot failure by spurious NMIs.

Add a diagnostic message in case we need to know whether early NMIs have
occurred and/or what happened to them.

Signed-off-by: Jun'ichi Nomura <junichi.nomura@nec.com>
Suggested-by: Borislav Petkov <bp@alien8.de>
Suggested-by: H. Peter Anvin <hpa@zytor.com>
Link: https://lore.kernel.org/lkml/20231130103339.GCZWhlA196uRklTMNF@fat_crate.local/
--
v2
  * Moved variable declaration and definition to the right place
    based on comments from Kirill.
    No functional changes.

diff --git a/arch/x86/boot/compressed/ident_map_64.c b/arch/x86/boot/compressed/ident_map_64.c
--- a/arch/x86/boot/compressed/ident_map_64.c
+++ b/arch/x86/boot/compressed/ident_map_64.c
@@ -390,4 +390,5 @@ void do_boot_page_fault(struct pt_regs *regs, unsigned long error_code)
 void do_boot_nmi_trap(struct pt_regs *regs, unsigned long error_code)
 {
 	/* Empty handler to ignore NMI during early boot */
+	spurious_nmi_count++;
 }
diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c
@@ -52,6 +52,7 @@ struct port_io_ops pio_ops;
 
 memptr free_mem_ptr;
 memptr free_mem_end_ptr;
+int spurious_nmi_count;
 
 static char *vidmem;
 static int vidport;
@@ -493,6 +494,12 @@ asmlinkage __visible void *extract_kernel(void *rmode, unsigned char *output)
 	/* Disable exception handling before booting the kernel */
 	cleanup_exception_handling();
 
+	if (spurious_nmi_count) {
+		error_putstr("Spurious early NMI ignored. Number of NMIs: 0x");
+		error_puthex(spurious_nmi_count);
+		error_putstr("\n");
+	}
+
 	return output + entry_offset;
 }
 
diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
--- a/arch/x86/boot/compressed/misc.h
+++ b/arch/x86/boot/compressed/misc.h
@@ -59,6 +59,7 @@ extern char _head[], _end[];
 /* misc.c */
 extern memptr free_mem_ptr;
 extern memptr free_mem_end_ptr;
+extern int spurious_nmi_count;
 void *malloc(int size);
 void free(void *where);
 void __putstr(const char *s);