linux-kernel - Re: [KVM PATCH 1/2] KVM: Directly inject interrupts via irqfd

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Date:	Thu, 22 Oct 2009 11:14:40 -0400
From:	Gregory Haskins <gregory.haskins@...il.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Gleb Natapov <gleb@...hat.com>,
	Gregory Haskins <ghaskins@...ell.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	alacrityvm-devel@...ts.sourceforge.net
Subject: Re: [KVM PATCH 1/2] KVM: Directly inject interrupts via irqfd

Avi Kivity wrote:
> On 10/21/2009 05:42 PM, Gregory Haskins wrote:
>> I believe Avi, Michael, et. al. were in agreement with me on that design
>> choice.  I believe the reason is that there is no good way to do EOI/ACK
>> feedback within the constraints of an eventfd pipe which would be
>> required for the legacy pin-type interrupts.  Therefore, we won't even
>> bother trying.  High-performance subsystems will use irqfd/msi, and
>> legacy emulation can use the existing injection code (which includes the
>> necessary feedback for ack/eoi).
>>
>>    
> 
> Right.  But we don't actually prevent anyone using non-msi with irqfd,
> which can trigger the bad lock usage from irq context, with a nice boom
> afterwards.  So we need to either prevent it during registration, or to
> gracefully handle it afterwards.
> 

Yeah, I was thinking about that after I initially responded to Gleb.

I am thinking something along these lines:

Provide a function that lets you query a GSI for whether it supports
LOCKLESS or not.  Then we can either do one of two things:

1) Check for the LOCKLESS attribute at irqfd registration, fail if not
present

2) Cache the LOCKLESS attribute in the irqfd structure, and either go
direct or defer to a workqueue depending on the flag.

Thoughts?
-Greg


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