virtio — archive

[PATCH v5 0/7] Introduce device group and device management

From: Michael S. Tsirkin <[email protected]>
To: [email protected]
Date: Tue, 17 May 2022 11:48:00 -0000

On Mon, May 16, 2022 at 09:08:34PM +0000, Parav Pandit wrote: > Hi Michael, > > > From: Michael S. Tsirkin <[email protected]> > > Sent: Sunday, May 15, 2022 11:24 AM > > [..] > > > > +\subsection{VIRTIO ADMIN DEVICE CAPS ACCEPT > > command}\label{sec:Basic > > > +Facilities of a Virtio Device / Admin command set / VIRTIO ADMIN > > > +DEVICE CAPS ACCEPT command} > > > + > > > +The VIRTIO_ADMIN_DEVICE_CAPS_ACCEPT command is used by the > > driver to acknowledge those admin capabilities it understands and wishes to > > use. > > > > > > ok so we have a protocol here, kind of like feature negotiation. Please write > > its description. > > e.g. is it ok to change accepted caps? when? can device change its caps etc > > etc etc. > > > > Avoiding this kind of spec work is exactly why me and jason keep telling you > > to consider just using features instead. Add a 64 bit admin features field to > > the PCI transport and be done with it. CCW and MMIO already have feature > > selector so it's trivial to add feature bits. > > > As we begin to scale with the device, adding more and more registers like this demands more on-device real estate to comply to the PCI standards. > > And therefore, things are queried/accessed rare or occasionally, are better accessed via a queue interface. > > One can argue that admin VQ is proposed only for the mgmt. functions so having this cfg register for PF is enough. > > However, AQ may find some usage in the VF/SF themselves down the road. > Hence, keeping the cap exchange transport this way is more optimal. > > Max has called out this AQ rationale in 4 or 5 points in the cover letter. Hmm. It's kind of a generic claim though. We never put devices on a diet trying to conserve registers. There is cost associated with this dance and that is driver boot time. I also don't really understand how you can claim you need to save memory like this and at the same time blindly add a more or less "just in case" misc config in the config space. So, not pretty. And as I said, you will need much more spec work to reach the level to which features are specified - and note we are not yet happy with how features are specified either! So it's a moving target. Maybe put this in features for now, and leave the whole capability thing for another day? -- MST