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/

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
@@ -387,7 +387,10 @@ void do_boot_page_fault(struct pt_regs *regs, unsigned long error_code)
         kernel_add_identity_map(address, end);
  }

+extern int spurious_nmi_count;
+
  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
@@ -357,6 +357,8 @@ unsigned long decompress_kernel(unsigned char *outbuf, unsigned long virt_addr,
         return entry;
  }

+int spurious_nmi_count;
+
  /*
   * The compressed kernel image (ZO), has been moved so that its position
   * is against the end of the buffer used to hold the uncompressed kernel
@@ -493,6 +495,12 @@ 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;
  }