Re: [PATCH v6 1/5] Introduce device group

[Jason Wang <jasowang@redhat.com>]

å 2022/8/3 14:10, Michael S. Tsirkin åé:
> On Wed, Aug 03, 2022 at 12:44:38PM +0800, Jason Wang wrote:
> > On Tue, Aug 2, 2022 at 9:42 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > I feel some of my latest review opened some questions that I don't have
> > > good answers for and might have felt a bit rambling.
> > > So to focus the discussion:
> > >
> > > On Sun, Jul 31, 2022 at 06:43:50PM +0300, Max Gurtovoy wrote:
> > > > +A device can be a member of one or more device groups.
> > > Presumably this is so we can e.g. create subfunctions inside a VF.
> > Then VF should have its own transport virtqueue. And subfunctions need
> > to be created there. If we don't all thing in PF, we may end up with
> > nesting issue when assign VF to the guest.
> > > A VF now is a member of a SRIOV and SIOV type groups and we
> > > can use type to distinguish between these.
> > >
> > > We should probably be explicit that each of these groups has to
> > > have a distinct group type then.
> > >
> > > And this raises the question: different types have different
> > > capabilities. So let's say admin queue is used to both
> > > control features for SRIOV VFs and to create SIOV SFs.
> > I don't get how the admin queue can be used to control VF features
> > considering VF has its capabilities. (SR-IOV lacks the ability to
> > provision a single VF).
> Well look at latest proposal, last patch controls VF features from PF.


Yes, so it works like previous MSI-X allocation which needs some care to 
prevent managed device from being probed before assigning features.

This is technically possible, but I'm not sure it is a good design. For 
example, what happens if the management change the feature while the a 
driver is using the managed device.


> > > I guess we'll have a feature bit to say "command to create
> > > SIOV SFs is supported" but how do we say that this command
> > > is only supported for VFs not SFs?
> > I think we should first answer if having VF and SF to be dealt with a
> > single type of virtqueue is a good idea. They have something in common
> > but they distinguish each other:
> >
> > - SF requires per virtual device lifecycle management
> > - SF requires a transport other than PCI
> > - SF requires more mediation in the software layer for presenting a
> > virtual device
> >
> > Using a single type of virtqueue may end up with complex design.
> > Having a dedicated queue for SF might be a better choice.
> And dedicated feature bits for commands thereof?


Only needed if we're using a single type of the queue.


>    For example, I imagine
> we could have commands to control the MAC of the group member. That is
> the same for SF and VF, yes? How do we avoid duplication for that?


In the transport vq, all configs (include mtu and features) were 
specified during the device creating command. It is not allowed to 
change mac afterwards. (If we need, the SF needs to be destroyed and 
created again with different configs).


> > > Do we just make features list a superset of what is supported and simply
> > > say in the spec which commands are legal with which group types?
> > >
> > >
> > > Jason Cornelia what do you think?
> > It looks to me it would be much more simpler if we use separated
> > virtqueues for SRIOV and SIOV.
> >
> > Thanks
> Then is it still helpful that we have the generic group type concept?


Not sure, I wonder if the implicit group can do here. E.g _F_SRIOV with 
_F_ADMIN_VQ menast SR-IOV group.


> I was hoping it will work so the same command can be used for VFs
> and SFs.


Yes, but the transport vq ties the mac and other configuration with the 
device creating. Not sure we can easily do the same for SR-IOV.

Thanks


> > > > +\item Self type (group identifier = 0) - this group has only one device in the group. Each virtio device is a member of at least one device group, the Self type group.
> > > Presumably, this is here so we can send commands that refer to the
> > > device itself as opposed to a group member (e.g. to
> > > PF as opposed to VF). Is that right?
> > >
> > > It's handy but again the problem here is, this refers to
> > > device as part of which group? Let's just drop this type?
> > >
> > >
> > > --
> > > MST