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SubjectRe: virtio-pci new configuration proposal
On Thu, 2011-11-03 at 12:28 +1030, Rusty Russell wrote: 
> On Wed, 02 Nov 2011 20:49:27 +0200, Sasha Levin <> wrote:
> > This is a proposal for a new layout of the virtio-pci config space.
> >
> > We will separate the current configuration into two: A virtio-pci common
> > configuration and a device specific configuration. This allows more flexibility
> > with adding features and makes usage easier, specifically in cases like the
> > ones in virtio-net where device specific configurations depend on device
> > specific features.
> Thanks for this Sasha. Several general comments:
> 1) How to we distinguish the two layouts? In theory, we need to do this
> forever. In practice we can deprecate the old layout in several
> years' time.

Old layouts won't have the new virtio-pci cap structure in their PCI
config space.

> 2) I don't think we want to turn the device-specific config into a
> linked list. We haven't needed variable-length config (yet!), and
> it's (slightly) more complex. That's also the part of the spec which
> is shared with non-PCI virtio implementations.

Variable length config wasn't used yet because space in the device
specific space was reserved for a feature even if that feature wasn't

For example, the MAC feature reserved 6 bytes in the config space for
the MAC even if VIRTIO_NET_F_MAC wasn't enabled. Here we can just avoid
having it pollute the config space until it's enabled.

I don't think it'll have any impact on non-PCI implementations since the
"pointers" are simply offsets from the beginning of the config space,
and are not PCI specific in any way.

> 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.



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