[tip: x86/entry] x86/traps: Document do_spurious_interrupt_bug()

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the x86/entry branch of tip:

Commit-ID:     d244d0e195bc12964bcf3b8eef45e715a7f203b0
Gitweb:        https://git.kernel.org/tip/d244d0e195bc12964bcf3b8eef45e715a7f203b0
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Tue, 25 Feb 2020 22:36:41 +01:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Thu, 27 Feb 2020 14:48:40 +01:00

x86/traps: Document do_spurious_interrupt_bug()

Add a comment which explains why this empty handler for a reserved vector
exists.

Requested-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Reviewed-by: Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>
Reviewed-by: Andy Lutomirski <luto@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200225220216.624165786@xxxxxxxxxxxxx

---
 arch/x86/kernel/traps.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 474b8cb..7ffb6f4 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -862,6 +862,25 @@ do_simd_coprocessor_error(struct pt_regs *regs, long error_code)
 dotraplinkage void
 do_spurious_interrupt_bug(struct pt_regs *regs, long error_code)
 {
+	/*
+	 * This addresses a Pentium Pro Erratum:
+	 *
+	 * PROBLEM: If the APIC subsystem is configured in mixed mode with
+	 * Virtual Wire mode implemented through the local APIC, an
+	 * interrupt vector of 0Fh (Intel reserved encoding) may be
+	 * generated by the local APIC (Int 15).  This vector may be
+	 * generated upon receipt of a spurious interrupt (an interrupt
+	 * which is removed before the system receives the INTA sequence)
+	 * instead of the programmed 8259 spurious interrupt vector.
+	 *
+	 * IMPLICATION: The spurious interrupt vector programmed in the
+	 * 8259 is normally handled by an operating system's spurious
+	 * interrupt handler. However, a vector of 0Fh is unknown to some
+	 * operating systems, which would crash if this erratum occurred.
+	 *
+	 * In theory this could be limited to 32bit, but the handler is not
+	 * hurting and who knows which other CPUs suffer from this.
+	 */
 }
 
 dotraplinkage void