On Tue, Jan 31, 2023 at 08:11:06PM +0100, Eugenio Perez Martin wrote: > On Tue, Jan 31, 2023 at 8:10 PM Eugenio Perez Martin > <epere...@redhat.com> 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? > > > > Thanks! > > > (Starting a separated thread to vhost-vdpa related use case) > > This could also work for vhost-vdpa if we ever decide to honor netdev > queues argument. It is totally ignored now, as opposed to the rest of > backends: > * vhost-kernel, whose tap device has the requested number of queues. > * vhost-user, that errors with ("you are asking more queues than > supported") if the vhost-user parent device has less queues than > requested (by vhost-user msg VHOST_USER_GET_QUEUE_NUM). > > One of the reasons for this is that device configuration space is > totally passthrough, with the values for mtu, rss conditions, etc. > This is not ideal, as qemu cannot check src and destination > equivalence and they can change under the feets of the guest in the > event of a migration. External tools are needed for this, duplicating > part of the effort. > > Start intercepting config space accesses and offering an emulated one > to the guest with this kind of adjustments is beneficial, as it makes > vhost-vdpa more similar to the rest of backends, making the surprise > on a change way lower. > > Thoughts? > > Thanks!
I agree here. -- MST