linux-kernel - Re: [KVM PATCH v4 2/2] kvm: add support for irqfd via eventfd-notification interface

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Message-ID: <4A02FD8D.50002@redhat.com>
Date:	Thu, 07 May 2009 18:26:05 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Gregory Haskins <ghaskins@...ell.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	viro@...IV.linux.org.uk, kvm@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [KVM PATCH v4 2/2] kvm: add support for irqfd via eventfd-notification
 interface

Gregory Haskins wrote:

  

>> This is my preferred option.  For a virtio-net-server in the kernel,
>> we'd service its eventfd in qemu, raising and lowering the pci
>> interrupt in the traditional way.
>>
>> But we'd still need to know when to lower the interrupt.  How?
>>     
>
> IIUC, isn't that  usually device/subsystem specific, and out of scope of
> the GSI delivery vehicle?  For instance, most devices I have seen with
> level ints have a register in their device register namespace for acking
> the int.  

Yes it is.

> As an aside, this is what causes some of the grief in dealing
> with shared interrupts like KVM pass-through and/or threaded-isrs:  
> There isn't a standardized way to ACK them.
>   

So we'd need a side channel to tell userspace to lower the irq.  Another 
eventfd likely.

Note we don't support device assignment for devices with shared interrupts.

> You may also see some generalization of masking/acking in things like
> the MSI-X table.  But again, this would be out of scope of the general
> GSI delivery path IIUC.
>
> I understand that there is a feedback mechanism in the ioapic model for
> calling back on acknowledgment of the interrupt.  But I am not sure what
> is how the real hardware works normally, and therefore I am not
> convinced that is something we need to feed all the way back (i.e. via
> irqfd or whatever).  In the interest of full disclosure, its been a few
> years since I studied the xAPIC docs, so I might be out to lunch on that
> assertion. ;)
>   

Right, that ack thing is completely internal, used for catching up on 
time drift, and for shutting down level triggered assigned interrupts.

-- 
error compiling committee.c: too many arguments to function

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