linux-kernel - Re: [PATCH v3] enable x2APIC without interrupt remapping under KVM

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Date:	Tue, 30 Jun 2009 12:36:01 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Avi Kivity <avi@...hat.com>
Cc:	Gleb Natapov <gleb@...hat.com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Sheng Yang <sheng@...ux.intel.com>,
	"kvm\@vger.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH v3] enable x2APIC without interrupt remapping under KVM

Avi Kivity <avi@...hat.com> writes:

> On 06/30/2009 10:08 PM, Eric W. Biederman wrote:
>>> Can you elaborate?  For kvm guests, the hardware is reasonably will implemented
>>> and if not we will fix it.  We need not cripple a feature just because some
>>> hardware is broken.
>>>      
>>
>> The short version is I don't know what work arounds we will ultimately
>> decide to deploy to work with real hardware.
>>
>> I have been seriously contemplating causing a cpu hot-unplug request
>> to fail if we are in ioapic mode and we have irqs routed to the cpu
>> that is being unplugged.
>>    
>
> Well, obviously we need to disassociate any irqs from such a cpu.  Could be done
> from the kernel or only enforced by the kernel.

Using the normal irq migration path we can move irqs off of a cpu reliably
there just aren't any progress guarantees.

>> Even with perfectly working hardware it is not possible in the general
>> case to migrate an ioapic irq from one cpu to another outside of an
>> interrupt handler without without risking dropping an interrupt.
>>    
>
> Can't you generate a spurious interrupt immediately after the migration?  An
> extra interrupt shouldn't hurt.

Nope.  The ioapics can't be told to send an interrupt.

>> There is no general way to know you have seen the last interrupt
>> floating around your system.  PCI ordering rules don't help because
>> the ioapics can potentially take an out of band channel.
>>    
>
> Can you describe the problem scenario? an ioapic->lapic message delivered to a
> dead cpu?

Dropped irqs..  Driver hangs because it is waiting for an irq.  Hardware
hangs because it is waiting for the cpu to process the irq.

Potentially we get a level triggered irq that is never acked by
the cpu that won't arm until the cpu send an ack, and we can't
send an ack from another cpu.

Eric
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