Messages in this thread Patch in this message | | | Date | Tue, 1 Apr 2014 13:30:21 +0100 (BST) | From | "Maciej W. Rozycki" <> | Subject | [PATCH] x86/apic: Reinstate error IRQ Pentium erratum 3AP workaround |
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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 compatibility[2]. The approach taken in the same as in lapic_setup_esr.
References:
[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@linux-mips.org> --- linux-apic-esr-irq-3ap.patch Index: linux-20140329-tp/arch/x86/kernel/apic/apic.c =================================================================== --- linux-20140329-tp.orig/arch/x86/kernel/apic/apic.c +++ linux-20140329-tp/arch/x86/kernel/apic/apic.c @@ -1989,7 +1989,8 @@ static inline void __smp_error_interrupt }; /* 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); ack_APIC_irq(); atomic_inc(&irq_err_count);
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