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
