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Subject: Re: [virtio-comment] RE: [PATCH v5 2/7] Introduce admin command set
On Wed, May 18 2022, Max Gurtovoy <mgurtovoy@nvidia.com> wrote: > On 5/17/2022 2:48 PM, Michael S. Tsirkin wrote: >> On Mon, May 16, 2022 at 09:08:34PM +0000, Parav Pandit wrote: >>> Hi Michael, >>> >>>> From: Michael S. Tsirkin <mst@redhat.com> >>>> 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? > > If you're not happy on the feature negotiation in the spec so please > don't insist we use this mechanism for admin capabilities. > > I don't want to postpone essential and basic definition to another day. > > We need to agree on it today (even yesterday). > >> Sorry, we will not agree on this today. And certainly not yesterday. I'll pull out of this discussion. If something is agreed, I'll vote on it.
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