[tip:x86/urgent] x86/apic: Reinstate error IRQ Pentium erratum 3AP workaround

From: tip-bot for Maciej W. Rozycki
Date: Tue Apr 01 2014 - 09:04:11 EST

Commit-ID: 023de4a09f571fad0af9691e4e437e14b68f05fb
Gitweb: http://git.kernel.org/tip/023de4a09f571fad0af9691e4e437e14b68f05fb
Author: Maciej W. Rozycki <macro@xxxxxxxxxxxxxx>
AuthorDate: Tue, 1 Apr 2014 13:30:21 +0100
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 1 Apr 2014 14:59:43 +0200

x86/apic: Reinstate error IRQ Pentium erratum 3AP workaround

A change introduced with commit 60283df7ac26a4fe2d56631ca2946e04725e7eaf
("x86/apic: Read Error Status Register correctly") removed a read from the
APIC ESR register made before writing to same required to retrieve the
correct error status on Pentium systems affected by the 3AP erratum[1]:

"3AP. Writes to Error Register Clears Register

PROBLEM: The APIC Error register is intended to only be read.
If there is a write to this register the data in the APIC Error
register will be cleared and lost.

IMPLICATION: There is a possibility of clearing the Error
register status since the write to the register is not
specifically blocked.

WORKAROUND: Writes should not occur to the Pentium processor
APIC Error register.

STATUS: For the steppings affected see the Summary Table of
Changes at the beginning of this section."

The steppings affected are actually: B1, B3 and B5.

To avoid this information loss this change avoids the write to
ESR on all Pentium systems where it is actually never needed;
in Pentium processor documentation ESR was noted read-only and
the write only required for future architectural

The approach taken is the same as in lapic_setup_esr().


[1] "Pentium Processor Family Developer's Manual", Intel Corporation,
1997, order number 241428-005, Appendix A "Errata and S-Specs for the
Pentium Processor Family", p. A-92,

[2] "Pentium Processor Family Developer's Manual, Volume 3: Architecture
and Programming Manual", Intel Corporation, 1995, order number
241430-004, Section 19.3.3. "Error Handling In APIC", p. 19-33.

Signed-off-by: Maciej W. Rozycki <macro@xxxxxxxxxxxxxx>
Cc: Richard Weinberger <richard@xxxxxx>
Link: http://lkml.kernel.org/r/alpine.LFD.2.11.1404011300010.27402@xxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
arch/x86/kernel/apic/apic.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index 53e2053..005ed3f 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1996,7 +1996,8 @@ static inline void __smp_error_interrupt(struct pt_regs *regs)

/* First tickle the hardware, only then report what went on. -- REW */
- apic_write(APIC_ESR, 0);
+ if (lapic_get_maxlvt() > 3) /* Due to the Pentium erratum 3AP. */
+ apic_write(APIC_ESR, 0);
v = apic_read(APIC_ESR);
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