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Subject: Re: virtio-pci new configuration proposal
From: Sasha Levin <levinsasha928@gmail.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>,
        linux-kernel <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        virtualization <virtualization@lists.linux-foundation.org>,
        Anthony Liguori <anthony@codemonkey.ws>
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Date: Fri, 04 Nov 2011 14:32:19 +0200
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On Fri, 2011-11-04 at 13:40 +0200, Michael S. Tsirkin wrote:
> On Fri, Nov 04, 2011 at 08:14:43PM +1030, Rusty Russell wrote:
> > > > 3) If we're changing the queue layout, it's a chance to fix a
> > > >    longstanding bug: let the guest notify the host of preferred
> > > >    queue size and alignment.
> > > 
> > > Yup, we can do that.
> 
> We don't need to change all of layout for that - just add another field
> in the common config structure to supply the alignment.

How would you do it without changing the layout? Add another optional
field at the end which will shift offsets based on whether the host and
guest support this new feature or not?

This leads to 3 different things which now shift config offsets around.

As you said, the PCI cap list was introduced both to save space (which
is not the motivation here), and because it's a very efficient and easy
way to manage optional features without requiring tricks which move
offsets around like we do now.

-- 

Sasha.