On Wed, Mar 08, 2023 at 11:33:45AM +0100, Maxime Coquelin wrote: > Hello Michael, > > On 2/1/23 12:20, Michael S. Tsirkin wrote: > > On Wed, Feb 01, 2023 at 12:14:18PM +0100, Maxime Coquelin wrote: > > > Thanks Eugenio for working on this. > > > > > > On 1/31/23 20:10, Eugenio Perez Martin wrote: > > > > Hi, > > > > > > > > The current approach of offering an emulated CVQ to the guest and map > > > > the commands to vhost-user is not scaling well: > > > > * Some devices already offer it, so the transformation is redundant. > > > > * There is no support for commands with variable length (RSS?) > > > > > > > > We can solve both of them by offering it through vhost-user the same > > > > way as vhost-vdpa do. With this approach qemu needs to track the > > > > commands, for similar reasons as vhost-vdpa: qemu needs to track the > > > > device status for live migration. vhost-user should use the same SVQ > > > > code for this, so we avoid duplications. > > > > > > > > One of the challenges here is to know what virtqueue to shadow / > > > > isolate. The vhost-user device may not have the same queues as the > > > > device frontend: > > > > * The first depends on the actual vhost-user device, and qemu fetches > > > > it with VHOST_USER_GET_QUEUE_NUM at the moment. > > > > * The qemu device frontend's is set by netdev queues= cmdline parameter > > > > in qemu > > > > > > > > For the device, the CVQ is the last one it offers, but for the guest > > > > it is the last one offered in config space. > > > > > > > > To create a new vhost-user command to decrease that maximum number of > > > > queues may be an option. But we can do it without adding more > > > > commands, remapping the CVQ index at virtqueue setup. I think it > > > > should be doable using (struct vhost_dev).vq_index and maybe a few > > > > adjustments here and there. > > > > > > > > Thoughts? > > > > > > I am fine with both proposals. > > > I think index remapping will require a bit more rework in the DPDK > > > Vhost-user library, but nothing insurmountable. > > > > > > I am currently working on a PoC adding support for VDUSE in the DPDK > > > Vhost library, and recently added control queue support. We can reuse it > > > if we want to prototype your proposal. > > > > > > Maxime > > > > > > > Thanks! > > > > > > > > > > technically backend knows how many vqs are there, last one is cvq... > > not sure we need full blown remapping ... > > > > Before VHOST_USER_PROTOCOL_F_STATUS was supported by qemu (very > recently, v7.2.0), we had no way for the backend to be sure the > frontend won't configure new queue pairs, this not not defined in the > spec AFAICT [0]. In DPDK Vhost library, we notify the application it can > start to use the device once the first queue pair is setup and enabled, > then we notify the application when new queues are ready to be > processed. In this case, I think we cannot deduce whether the queue is a > data or a control queue when it is setup. > > When VHOST_USER_PROTOCOL_F_STATUS is supported, we know no more queues > will be configured once the DRIVER_OK status is set. In this case, we > can deduce the last queue setup will be the control queue at DRIVER_OK > time if the number of queues is odd. > > Using index remapping, we would know directly at queue setup time > whether this is a data or control queue based on its index value, > i.e. if the index equals to max queue index supported by the backend. > But thinking at it again, we may at least back this with a protocol > feature to avoid issues with legacy backends. > > I hope it clarifies, let me know if anything unclear. > > Thanks, > Maxime > > [0]: > https://elixir.bootlin.com/qemu/latest/source/docs/interop/vhost-user.rst
OK maybe document this. -- MST
