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From: (Eric W. Biederman)
To: Andrew Morton <>
	Zwane Mwaikambo <>,
	Ashok Raj <>, Ingo Molnar <>,
	"Lu, Yinghai" <>,
	Natalie Protasevich <>, Andi Kleen <>,
	"Siddha, Suresh B" <>,
	Linus Torvalds <>
Subject: [PATCH] x86_64 irq:  Document what works and why on ioapics.
Date: Fri, 23 Feb 2007 05:01:38 -0700	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <> (Eric W. Biederman's message of "Fri, 23 Feb 2007 04:46:20 -0700")

After writing this up and sending out the email it occured to me this
information should be kept someplace a little more permanent, so the
next person who cares won't have to get a huge pile of test machines
and test to understand what doesn't work. 

A bunch of this is in my other changelog entries in the patches I
just posted but not all of it.

Signed-off-by: Eric W. Biederman <>
 Documentation/x86_64/IO-APIC-what-works.txt |  109 +++++++++++++++++++++++++++
 1 files changed, 109 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/x86_64/IO-APIC-what-works.txt

diff --git a/Documentation/x86_64/IO-APIC-what-works.txt b/Documentation/x86_64/IO-APIC-what-works.txt
new file mode 100644
index 0000000..40fa61f
--- /dev/null
+++ b/Documentation/x86_64/IO-APIC-what-works.txt
@@ -0,0 +1,109 @@
+23 Feb 2007
+Ok. This is just an email to summarize my findings after investigating
+the ioapic programming.
+The ioapics on the E75xx chipset do have issues if you attempt to
+reprogramming them outside of the irq handler.  I have on several
+instances caused the state machine to get stuck such that an
+individual ioapic entry was no longer capable of delivering
+interrupts.  I suspect the remote IRR bit was set stuck on such that
+switch the irq to edge triggered and back to level triggered would not
+clear it but I did not confirm this.  I just know that I was switching
+the irq to between level and edge triggered with the irq masked
+and the irq did not fire.
+The ioapics on the AMD 8xxx chipset do have issues if you attempt
+to reprogram them outside of the irq handler.  I would up with 
+remote IRR set and never clearing.  But by temporarily switching
+the irq to edge triggered while it was masked I could clear
+this condition.
+I could not hit verifiable bugs in the ioapics on the Nforce4
+chipset.  It's amazing one part of that chipset that I can't find
+issues with.
+I did find an algorithm that will work successfully for migrating
+IRQs in process context if you have an ioapic that will follow pci
+ordering rules.  In particulars the properties that the algorithm
+depend on are reads guaranteeing that outstanding writes are flushed,
+and in this context irqs in flight are considered writes.  I have
+assumed that to devices outside of the cpu asic the cpu and the local
+apic appear as the same device.
+The algorithm was:
+- Be running with interrupts enabled in process context.
+- Mask the ioapic.
+- Read the ioapic to flush outstanding reads to the local apic.
+- Read the local apic to flush outstanding irqs to be send the cpu.
+- Now that all of the irqs have been delivered and the irq is masked
+  that irq is finally quiescent.
+- With the irq quiescent it is safe to reprogram interrupt controller
+  and the irq reception data structures.
+There were a lot more details but that was the essence.
+What I discovered was that except on the nforce chipset masking the
+ioapic and then issue a read did not behave as if the interrupts were
+flushed to the local apic. 
+I did not look close enough to tell if local apics suffered from this
+issue.  With local apics at least a read was necessary before you
+could guarantee the local apic would deliver pending irqs.  A work
+around on the local apics is to simply issue a low priority interrupt
+as an IPI and wait for it to be processed.  This guarantees that all
+higher priority interrupts have been flushed from the apic, and that
+the local apic has processed interrupts.
+For ioapics because they cannot be stimulated to send any irq by
+stimulation from the cpu side not similar work around was possible.
+** Conclusions.
+*IRQs must be reprogramed in interrupt context.
+The result of this is investigation is that I am convinced we need
+to perform the irq migration activities in interrupt context although
+I am not convinced it is completely safe.  I suspect multiple irqs
+firing closely enough to each other may hit the same issues as
+migrating irqs from process context.  However the odds are on our
+side, when we are in irq context.
+The reasoning for this is simply that.
+- Before we reprogram a level triggered irq it's remote irr bit
+  must be cleared by the irq being acknowledged before the can be
+  safely reprogrammed.
+- There is no generally effective way short of receiving an additional
+  irq to ensure that the irq handler has run.  Polling the ioapics
+  remote irr bit does not work.
+* The CPU hotplug code is currently very buggy.
+Irq migration in the cpu hotplug case is a serious problem.  If we can
+only safely migrate irqs from interrupt context and we cannot control
+when those interrupts fire, then we cannot bound the amount of time it
+will take to migrate the irqs away from a cpu.   The current cpu
+hotplug code currently calls chip->set_affinity directly which is
+wrong, as it does not take the necessary locks, and it does not
+attempt to delay execution until we are in process context.
+* Only an additional irq can signal the completion of an irq movement.
+The attempt to rebuild the irq migration code from first principles
+did bear some fruit.  I asked the question: "When is it safe to tear
+down the data structures for irq movement?".  The only answer I have
+is when I have received an irq provably from after the irq was
+reprogrammed.  This is because the only way I can reliably synchronize
+with irq delivery from an apic is to receive an additional irq.
+Currently this is a problem both for cpu hotplug on x86_64 and i386
+and for general irq migration on x86_64.

  reply	other threads:[~2007-02-23 12:02 UTC|newest]

Thread overview: 71+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <>
2007-01-22 17:14 ` System crash after "No irq handler for vector" linux 2.6.19 Eric W. Biederman
     [not found]   ` <>
2007-01-23 12:18     ` Eric W. Biederman
     [not found]       ` <>
2007-01-31  8:39         ` Eric W. Biederman
     [not found]           ` <>
2007-02-01  5:56             ` [PATCH] x86_64: Survive having no irq mapping for a vector Eric W. Biederman
2007-02-01  5:59             ` System crash after "No irq handler for vector" linux 2.6.19 Eric W. Biederman
2007-02-01  7:20             ` Eric W. Biederman
     [not found]               ` <>
2007-02-02 18:02                 ` Eric W. Biederman
     [not found]                   ` <>
2007-02-02 18:32                     ` Eric W. Biederman
2007-02-03  0:31                     ` [PATCH 1/2] x86_64 irq: Simplfy __assign_irq_vector Eric W. Biederman
2007-02-03  0:35                       ` [PATCH 2/2] x86_64 irq: Handle irqs pending in IRR during irq migration Eric W. Biederman
2007-02-03  1:05                         ` Andrew Morton
2007-02-03  1:39                           ` Eric W. Biederman
2007-02-03  2:01                             ` Andrew Morton
2007-02-03  7:32                           ` Arjan van de Ven
2007-02-03  7:55                             ` Eric W. Biederman
2007-02-03 14:31                               ` l.genoni
2007-02-03 10:01                         ` Andi Kleen
2007-02-03 10:22                           ` Eric W. Biederman
2007-02-03 10:26                             ` Andi Kleen
2007-02-06  7:36                         ` Ingo Molnar
2007-02-06  8:57                           ` Eric W. Biederman
     [not found]                           ` <>
2007-02-06 22:05                             ` Eric W. Biederman
2007-02-06 22:16                           ` Eric W. Biederman
2007-02-06 22:25                             ` Ingo Molnar
2007-02-07  2:33                               ` Eric W. Biederman
2007-02-08 11:48                               ` Eric W. Biederman
2007-02-08 20:19                                 ` Eric W. Biederman
2007-02-09  6:40                                   ` Eric W. Biederman
2007-02-10 23:52                                     ` What are the real ioapic rte programming constraints? Eric W. Biederman
2007-02-11  5:57                                       ` Zwane Mwaikambo
2007-02-11 10:20                                         ` Eric W. Biederman
2007-02-11 16:16                                           ` Zwane Mwaikambo
2007-02-11 22:01                                             ` Eric W. Biederman
2007-02-12  1:05                                               ` Zwane Mwaikambo
2007-02-12  4:51                                                 ` Eric W. Biederman
2007-02-23 10:51                                                   ` Conclusions from my investigation about ioapic programming Eric W. Biederman
2007-02-23 11:10                                                     ` [PATCH 0/14] x86_64 irq related fixes and cleanups Eric W. Biederman
2007-02-23 11:11                                                       ` [PATCH 01/14] x86_64 irq: Simplfy __assign_irq_vector Eric W. Biederman
2007-02-23 11:13                                                         ` [PATCH 02/14] irq: Remove set_native_irq_info Eric W. Biederman
2007-02-23 11:15                                                           ` [PATCH 03/14] x86_64 irq: Kill declaration of removed array, interrupt Eric W. Biederman
2007-02-23 11:16                                                             ` [PATCH 04/14] x86_64 irq: Remove the unused vector parameter from ioapic_register_intr Eric W. Biederman
2007-02-23 11:19                                                               ` [PATCH 05/14] x86_64 irq: Refactor setup_IO_APIC_irq Eric W. Biederman
2007-02-23 11:20                                                                 ` [PATCH 06/14] x86_64 irq: Simplfiy the set_affinity logic Eric W. Biederman
2007-02-23 11:23                                                                   ` [PATCH 07/14] x86_64 irq: In __DO_ACTION perform the FINAL action for every entry Eric W. Biederman
2007-02-23 11:26                                                                     ` [PATCH 08/14] x86_64 irq: Use NR_IRQS not NR_IRQ_VECTORS Eric W. Biederman
2007-02-23 11:32                                                                       ` [PATCH 09/14] x86_64 irq: Begin consolidating per_irq data in structures Eric W. Biederman
2007-02-23 11:35                                                                         ` [PATCH 10/14] x86_64 irq: Simplify assign_irq_vector's arguments Eric W. Biederman
2007-02-23 11:36                                                                           ` [PATCH 11/14] x86_64 irq: Remove unnecessary irq 0 setup Eric W. Biederman
2007-02-23 11:38                                                                             ` [PATCH 12/14] x86_64 irq: Add constants for the reserved IRQ vectors Eric W. Biederman
2007-02-23 11:40                                                                               ` [PATCH 13/14] x86_64 irq: Safely cleanup an irq after moving it Eric W. Biederman
2007-02-25 11:53                                                                                 ` Mika Penttilä
2007-02-25 12:09                                                                                   ` Eric W. Biederman
2007-02-23 11:46                                                                               ` [PATCH 14/14] genirq: Mask irqs when migrating them Eric W. Biederman
2007-02-23 12:01                                                                                 ` Eric W. Biederman [this message]
2007-02-24  2:06                                                                                 ` Siddha, Suresh B
2007-02-27 20:26                                                                                   ` Andrew Morton
2007-02-27 20:41                                                                                     ` Eric W. Biederman
2007-02-25 10:43                                                                               ` [PATCH 12/14] x86_64 irq: Add constants for the reserved IRQ vectors Pavel Machek
2007-02-25 11:15                                                                                 ` Eric W. Biederman
2007-02-25 19:48                                                                                   ` Pavel Machek
2007-02-25 21:01                                                                                     ` Eric W. Biederman
2007-02-25 21:13                                                                                       ` Pavel Machek
2007-02-23 16:48                                                     ` Conclusions from my investigation about ioapic programming Jeff V. Merkey
2007-02-23 18:10                                                       ` Eric W. Biederman
2007-02-23 17:48                                                         ` Jeff V. Merkey
2007-02-24  4:05                                                           ` Eric W. Biederman
2007-02-24  5:44                                                             ` Jeffrey V. Merkey
2007-02-23 17:48                                                         ` Jeff V. Merkey
     [not found]                                           ` <>
2007-02-11 21:36                                             ` What are the real ioapic rte programming constraints? Eric W. Biederman
2007-02-03  9:50                       ` [PATCH 1/2] x86_64 irq: Simplfy __assign_irq_vector Andi Kleen
2007-02-03  0:40                     ` System crash after "No irq handler for vector" linux 2.6.19 Eric W. Biederman

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