Re: [virtio-dev] Backend libraries for VirtIO device emulation

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

* Alex Bennée (alex.bennee@linaro.org) wrote:
> Hi,
> 
> So the context of my question is what sort of common software layer is
> required to implement a virtio backend entirely in userspace?
> 
> Currently most virtio backends are embedded directly in various VMMs
> which emulate a number of devices as well as deal with handling devices
> that are vhost aware and link with the host kernel. However there seems
> to be a growing interest in having backends implemented in separate
> processes, potentially even hosted in other guest VMs.
> 
> As far as I can tell there is a lot of duplicated effort in handling the
> low level navigation of virt queues and buffers. QEMU has code in
> hw/virtio as well as contrib/libvhost-user which is used by the recent
> virtiofsd daemon. kvm-tool has a virtio subdirectory that implements a
> similar set of functionality for it's emulation. The Rust-vmm project
> has libraries for implementing the device traits.
> 
> Another aspect to this is the growing interest in carrying virtio over
> other hypervisors. I'm wondering if there is enough abstraction possible
> to have a common library that is hypervisor agnostic? Can a device
> backend be emulated purely with some shared memory and some sockets for
> passing messages/kicks from/to the VMM which then deals with the hypervisor
> specifics of the virtio-transport?

It's a little tricky because it has to interface tightly with the way
that the memory-mapping works for the hypervisor, so that the external
process can access the memory of the queues.
QEMU's vhost-user has a fair amount of code for handling the mappings,
dirty logging for migration, iommu's and things like reset (which is
pretty hairy, and probably needs more work).

Dave

> Thoughts?
> 
> -- 
> Alex Bennée
> 
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--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK