Re: [KVM PATCH v4 3/3] kvm: add iosignalfd support

From: Gregory Haskins <ghaskins@novell.com>
To: Mark McLoughlin <markmc@redhat.com>
Cc: Avi Kivity <avi@redhat.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Davide Libenzi <davidel@xmailserver.org>,
	mtosatti@redhat.com
Subject: Re: [KVM PATCH v4 3/3] kvm: add iosignalfd support
Date: Wed, 27 May 2009 13:40:13 -0400	[thread overview]
Message-ID: <4A1D7AFD.40004@novell.com> (raw)
In-Reply-To: <1243445144.16318.15.camel@blaa>

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Mark McLoughlin wrote:
> On Wed, 2009-05-27 at 15:11 +0300, Avi Kivity wrote:
>
>   
>> Multiple cookies on the same address are required by virtio.  You can't 
>> mux since the data doesn't go anywhere.
>>
>> Virtio can survive by checking all rings on a notify, and we can later 
>> add a mechanism that has a distinct address for each ring, but let's see 
>> if we can cope with multiple cookies.  Mark?
>>     
>
> Trying to catch up, but you're talking about replacing virtio-pci
> QUEUE_NOTIFY handling with iosignalfd ?
>
> For a perfect replacement, what you really need is to be able to
> register multiple cookies per address range, but only have them trigger
> if the written data matches a provided value.
>   

Hmm..thats an interesting idea.  To date, the "cookie" has really been
for identifying the proper range selected for deassignment.  I never
thought of using it as an actual trigger value at run-time.

> If the data is lost, virtio has no way of knowing which queue is being
> notified, so we either end up with per-device, rather than per-queue,
> notifications (probably not too bad for net, at least) or a different
> notify address per queue (limiting the number of queues per device).
>   

The addr-per-queue is how I was envisioning it, but the trigger value
concept hadn't occurred to me.  I could make this an option during
assignment (e.g. "COOKIE" flag means only trigger on writes of the
provided cookie, otherwise trigger on any write).  Sound good?

Thanks Mark,
-Greg

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