Re: [PATCH v8] introduce vfio-user protocol specification

[John Levon <levon@movementarian.org>]

On Tue, May 11, 2021 at 11:09:53AM +0100, Stefan Hajnoczi wrote:

> > > > +* *sub-regions* is the array of Sub-Region IO FD info structures
> > > > +
> > > > +The reply message will additionally include at least one file descriptor in the
> > > > +ancillary data. Note that more than one sub-region may share the same file
> > > > +descriptor.
> > > 
> > > How does this interact with the maximum number of file descriptors,
> > > max_fds? It is possible that there are more sub-regions than max_fds
> > > allows...
> > 
> > I think this would just be a matter of the client advertising a reasonably large
> > enough size for max_msg_fds. Do we need to worry about this?
> 
> vhost-user historically only supported passing 8 fds and it became a
> problem there.
> 
> I can imagine devices having 10s to 100s of sub-regions (e.g. 64 queue
> doorbells). Probably not 1000s.
> 
> If I was implementing a server I would check the negotiated max_fds and
> refuse to start the vfio-user connection if the device has been
> configured to require more sub-regions. Failing early and printing an
> error would allow users to troubleshoot the issue and re-configure the
> client/server.
> 
> This seems okay but the spec doesn't mention it explicitly so I wanted
> to check what you had in mind.

Not for the spec, but I filed https://github.com/nutanix/libvfio-user/issues/489
to track this on the library side. Thanks.

> Fleshing out irqs sounds like a 1.0 milestone to me. It will definitely
> be necessary but for now this can be dropped.

I could be wrong, and probably am, but I believe we're basically fine for IRQs
right now, until we want to support servers on separate hosts where we'll
obviously have to re-introduce something like the VM_INTERRUPT message.

> > > > +VFIO_USER_DEVICE_RESET
> > > > +----------------------
> > > 
> > > Any requirements for how long VFIO_USER_DEVICE_RESET takes to complete?
> > > In some cases a reset involves the server communicating with other
> > > systems or components and this can take an unbounded amount of time.
> > > Therefore this message could hang. For example, if a vfio-user NVMe
> > > device was accessing data on a hung NFS export and there were I/O
> > > requests in flight that need to be aborted.
> > 
> > I'm not sure this is something we could put in the generic spec. Perhaps a
> > caveat?
> 
> It's up to you whether you want to discuss this in the spec or let
> client implementors figure it out themselves. Any vfio-user message can
> take an unbounded amount of time and we could assume that readers will
> think of this.

I'm going to start an "implementation notes" section.

regards
john