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

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Date:	Wed, 27 May 2009 17:25:44 +0000
From:	Mark McLoughlin <markmc@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Gregory Haskins <ghaskins@...ell.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Davide Libenzi <davidel@...ilserver.org>, mtosatti@...hat.com
Subject: Re: [KVM PATCH v4 3/3] kvm: add iosignalfd support

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.

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).

Cheers,
Mark.

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