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

tip-bot2 for Thomas Gleixner
2020-02-27T14:16:13Z

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@linutronix.de>
AuthorDate: Tue, 25 Feb 2020 22:36:41 +01:00
Committer: Thomas Gleixner <tglx@linutronix.de>
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@redhat.com>

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

---
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