Re: RFC: New device for zero-copy VM memory access

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* geoff@hostfission.com (geoff@hostfission.com) wrote:
> 
> 
> On 2019-11-03 21:10, geoff@hostfission.com wrote:
> > On 2019-11-01 02:52, Dr. David Alan Gilbert wrote:
> > > * geoff@hostfission.com (geoff@hostfission.com) wrote:
> > > > 
> > > > 
> > > > On 2019-11-01 01:52, Peter Maydell wrote:
> > > > > On Thu, 31 Oct 2019 at 14:26, <geoff@hostfission.com> wrote:
> > > > > > As the author of Looking Glass, I also have to consider the
> > > > > > maintenance
> > > > > > and the complexity of implementing the vhost protocol into the
> > > > > > project.
> > > > > > At this time a complete Porthole client can be implemented in 150
> > > > > > lines
> > > > > > of C without external dependencies, and most of that is boilerplate
> > > > > > socket code. This IMO is a major factor in deciding to avoid
> > > > > > vhost-user.
> > > > >
> > > > > This is essentially a proposal that we should make our project and
> > > > > code more complicated so that your project and code can be simpler.
> > > > > I hope you can see why this isn't necessarily an argument that will hold
> > > > > very much weight for us :-)
> > > > 
> > > > Certainly, I do which is why I am still going to see about using
> > > > vhost,
> > > > however, a device that uses vhost is likely more complex then
> > > > the device
> > > > as it stands right now and as such more maintenance would be
> > > > involved on
> > > > your end also. Or have I missed something in that vhost-user can
> > > > be used
> > > > directly as a device?
> > > 
> > > The basic vhost-user stuff isn't actually that hard;  if you aren't
> > > actually shuffling commands over the queues you should find it pretty
> > > simple - so I think your assumption about it being simpler if you
> > > avoid
> > > it might be wrong.  It might be easier if you use it!
> > 
> > I have been looking into this and I am yet to find some decent
> > documentation or a simple device example I can use to understand how to
> > create such a device. Do you know of any reading or examples I can
> > obtain
> > on how to get an initial do nothing device up and running?
> > 
> > -Geoff
> 
> Scratch that, the design just solidified for me and I am now making
> progress, however it seems that vhost-user can't do what we need here:
> 
> 1) I dont see any way to recieve notification of socket disconnection, in
> our use case the client app needs to be able to be (re)connected
> dynamically. It might be possible to get this event by registering it on
> the chardev manually but this seems like it would be a kludge.

My understanding was that someone added support for reconnection of
vhost-user;  I'm not sure of the detail - cc'ing in Maxime and
Marc-Andre.

> 2) I don't see any method of notifying the vhost-user client of the
> removal of a shared memory mapping. Again, these may not be persistently
> mapped in the guest as we have no control over the buffer allocation, and
> as such, we need a method to notify the client that the mapping has become
> invalid.
> 
> 3) VHOST_USER_SET_MEM_TABLE is a one time request, again this breaks our
> usage as we need to change this dynamically at runtime.

I've seen (3) being sent multiple times (It's messy but it happens); so
I think that fixes (2) as well for you.

Dave

> Unless there are viable solutions to these problems there is no way that
> vhost-user can be used for this kind of a device.
> 
> -Geoff
> 
> > 
> > > 
> > > Dave
> > > 
> > > > >
> > > > > thanks
> > > > > -- PMM
> > > --
> > > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK