Re: [PATCH RFC v4 00/16] qemu: towards virtio-1 host support

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Thu, 27 Nov 2014 17:42:11 +0200
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Thu, Nov 27, 2014 at 04:31:39PM +0100, Cornelia Huck wrote:
> > On Thu, 27 Nov 2014 17:24:22 +0200
> > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > 
> > > On Thu, Nov 27, 2014 at 04:16:33PM +0100, Cornelia Huck wrote:
> > > > Yet another version of the virtio-1 support patches.
> > > > 
> > > > This one has seen some (very) light testing with the virtio-1 guest
> > > > support patches currently on vhost-next.
> > > > 
> > > > Changes from v3:
> > > > 
> > > > - Add support for FEATURES_OK. We refuse to set features after the
> > > >   driver has set this in the status field, and we allow to fail
> > > >   setting the status if the features are inconsistent.
> > > > - Add missing virtio-1 changes for virtio-net (header size and mac).
> > > > - Dropped setting the VERSION_1 bit for virtio-blk: There's still
> > > >   some stuff missing.
> > > > 
> > > > For virtio-blk, we need to validate the feature bits if version 1 is
> > > > negotiated: some legacy features are not allowed in that case. I'm not
> > > > quite sure how to handle this, though. We could use the new
> > > > validate_features callback to verify that the driver negotiated a
> > > > sensible feature set, but that would require us to offer a superset
> > > > of legacy and version 1 bits, which feels wrong. Any ideas?
> > > 
> > > No, that's violating the spec.
> > > I think the simplest way is to have separate features and
> > > legacy_features fields.  Present the correct one depending on which
> > > revision was negotiated.
> > 
> > But revisions are a virtio-ccw only thing - what can other transports
> > do here?
> 
> Other transports have different ways to deal with this.
> For example virtio pci exposes a legacy header and
> a modern header. Legacy header will expose old features,
> modern one - new features.
> 
> mmio simply does not support transitional devices.
> So qemu user will have to specify virtio 1.0 or 0.9 for mmio.
> 
> Other transports are out of virtio 1.0 spec so
> they just use legacy features.
> 
> > The basic problem is that we decide via a feature bit that
> > needs to be negotiated which feature bits we want to present.
> 
> Consider wce as one example.  This is not needed for modern guests, so
> we can just mask it from modern feature mask.  Consider virtio blk scsi
> commands as another example.  this feature is not supported in virtio
> 1.0, so we must mask it from modern feature mask.
> 
> Seems the same handling works in all cases?

This was just what I was talking about...

> 
> 
> > pci and
> > mmio don't have a way to know whether the driver wants to use 1.0 or
> > legacy prior to feature negotiation, do they?
> 
> pci does. mmio doesn't but it does not want to support transitional
> devices.
> 

So we should have a per-device callback into the transport layer, say
check_legacy()?

For ccw, this would check for the negotiated revision; for mmio, it
could check a device property configured with the device; and for pci,
whatever the mechanism is there :)

A transport not implementing this callback is simply considered
legacy-only.