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 2019-11-04 22:55, Dr. David Alan Gilbert wrote:
> > * 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.
> 
> Yes, but it's ignored.
> 
>     /*
>      * For non-vring specific requests, like VHOST_USER_SET_MEM_TABLE,
>      * we just need send it once in the first time. For later such
>      * request, we just ignore it.
>      */
>     if (vhost_user_one_time_request(msg->hdr.request) && dev->vq_index != 0)
> {
>          msg->hdr.flags &= ~VHOST_USER_NEED_REPLY_MASK;
>          return 0;
>     }

Curious.  I could swear I'd already dealt with multiple copies of this
message coming over vhost-user and having to deal with it when it did.
But now I'm confused, isn't vq_index a unique number per queue, so is
this really stopping it happening multiple times, or just making sure it
only happens for the first queue?

Dave

> > 
> > 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
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK