Re: Outline for VHOST_USER_PROTOCOL_F_VDPA

[Michael S. Tsirkin]

On Tue, Sep 29, 2020 at 09:57:51AM +0100, Stefan Hajnoczi wrote:
> On Tue, Sep 29, 2020 at 02:09:55AM -0400, Michael S. Tsirkin wrote:
> > On Mon, Sep 28, 2020 at 10:25:37AM +0100, Stefan Hajnoczi wrote:
> > > Why extend vhost-user with vDPA?
> > > ================================
> > > Reusing VIRTIO emulation code for vhost-user backends
> > > -----------------------------------------------------
> > > It is a common misconception that a vhost device is a VIRTIO device.
> > > VIRTIO devices are defined in the VIRTIO specification and consist of a
> > > configuration space, virtqueues, and a device lifecycle that includes
> > > feature negotiation. A vhost device is a subset of the corresponding
> > > VIRTIO device. The exact subset depends on the device type, and some
> > > vhost devices are closer to the full functionality of their
> > > corresponding VIRTIO device than others. The most well-known example is
> > > that vhost-net devices have rx/tx virtqueues and but lack the virtio-net
> > > control virtqueue. Also, the configuration space and device lifecycle
> > > are only partially available to vhost devices.
> > > 
> > > This difference makes it impossible to use a VIRTIO device as a
> > > vhost-user device and vice versa. There is an impedance mismatch and
> > > missing functionality. That's a shame because existing VIRTIO device
> > > emulation code is mature and duplicating it to provide vhost-user
> > > backends creates additional work.
> > 
> > 
> > The biggest issue facing vhost-user and absent in vdpa is
> > backend disconnect handling. This is the reason control path
> > is kept under QEMU control: we do not need any logic to
> > restore control path data, and we can verify a new backend
> > is consistent with old one.
> 
> I don't think using vhost-user with vDPA changes that. The VMM still
> needs to emulate a virtio-pci/ccw/mmio device that the guest interfaces
> with. If the device backend goes offline it's possible to restore that
> state upon reconnection. What have I missed?

The need to maintain the state in a way that is robust
against backend disconnects and can be restored.

> Regarding reconnection in general, it currently seems like a partially
> solved problem in vhost-user. There is the "Inflight I/O tracking"
> mechanism in the spec and some wording about reconnecting the socket,
> but in practice I wouldn't expect all device types, VMMs, or device
> backends to actually support reconnection. This is an area where a
> uniform solution would be very welcome too.

I'm not aware of big issues. What are they?

> There was discussion about recovering state in muser. The original idea
> was for the muser kernel module to host state that persists across
> device backend restart. That way the device backend can go away
> temporarily and resume without guest intervention.
> 
> Then when the vfio-user discussion started the idea morphed into simply
> keeping a tmpfs file for each device instance (no special muser.ko
> support needed anymore). This allows the device backend to resume
> without losing state. In practice a programming framework is needed to
> make this easy and safe to use but it boils down to a tmpfs mmap.
> 
> > > If there was a way to reuse existing VIRTIO device emulation code it
> > > would be easier to move to a multi-process architecture in QEMU. Want to
> > > run --netdev user,id=netdev0 --device virtio-net-pci,netdev=netdev0 in a
> > > separate, sandboxed process? Easy, run it as a vhost-user-net device
> > > instead of as virtio-net.
> > 
> > Given vhost-user is using a socket, and given there's an elaborate
> > protocol due to need for backwards compatibility, it seems safer to
> > have vhost-user interface in a separate process too.
> 
> Right, with vhost-user only the virtqueue processing is done in the
> device backend. The VMM still has to do the virtio transport emulation
> (pci, ccw, mmio) and vhost-user connection lifecycle, which is complex.

IIUC all vfio user does is add another protocol in the VMM,
and move code out of VMM to backend.

Architecturally I don't see why it's safer.

Something like multi-process patches seems like a way to
add defence in depth by having a process in the middle,
outside both VMM and backend.

> Going back to Marc-André's point, why don't we focus on vfio-user so the
> entire device can be moved out of the VMM?
> 
> Stefan

The fact that vfio-user adds a kernel component is one issue.

-- 
MST