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

[geoff@hostfission.com]

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;
     }

> 
> 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