Re: [virtio-dev] Memory sharing device

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 12 Feb 2019 11:25:47 +0000
"Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:

> * Roman Kiryanov (rkir@google.com) wrote:
> > > > Our long term goal is to have as few kernel drivers as possible and to move
> > > > "drivers" into userspace. If we go with the virtqueues, is there
> > > > general a purpose
> > > > device/driver to talk between our host and guest to support custom hardware
> > > > (with own blobs)?  
> > >
> > > The challenge is to answer the following question:
> > > how to do this without losing the benefits of standartization?  
> > 
> > We looked into UIO and it still requires some kernel driver to tell
> > where the device is, it also has limitations on sharing a device
> > between processes. The benefit of standardization could be in avoiding
> > everybody writing their own UIO drivers for virtual devices.
> > 
> > Our emulator uses a battery, sound, accelerometer and more. We need to
> > support all of this. I looked into the spec, "5 Device types", and
> > seems "battery" is not there. We can invent our own drivers but we see
> > having one flexible driver is a better idea.  
> 
> Can you group these devices together at all in their requirements?
> For example, battery and accelerometers (to me) sound like low-bandwidth
> 'sensors' with a set of key,value pairs that update occasionally
> and a limited (no?) amount of control from the VM->host.
> A 'virtio-values' device that carried a string list of keys that it
> supported might make sense and be enough for at least two of your
> device types.

Maybe not a 'virtio-values' device -- but a 'virtio-sensors' device
looks focused enough without being too inflexible. It can easily
advertise its type (battery, etc.) and therefore avoid the mismatch
problem that a too loosely defined device would be susceptible to.

> > Yes, I realize that a guest could think it is using the same device as
> > the host advertised (because strings matched) while it is not. We
> > control both the host and the guest and we can live with this.

The problem is that this is not true for the general case if you have a
standardized device type. It must be possible in theory to switch to an
alternative implementation of the device or the driver, as long as they
conform to the spec. I think a more concretely specified device type
(like the suggested virtio-values or virtio-sensors) is needed for that.