Re: [PATCH v4 00/15] Dynamically switch to vhost shadow virtqueues at vdpa net migration

[Alvaro Karsz <alvaro.karsz@solid-run.com>]

>
> It's possible to migrate vdpa net devices if they are shadowed from the
> start.  But to always shadow the dataplane is to effectively break its host
> passthrough, so its not efficient in vDPA scenarios.
>
> This series enables dynamically switching to shadow mode only at
> migration time.  This allows full data virtqueues passthrough all the
> time qemu is not migrating.
>
> In this series only net devices with no CVQ are migratable.  CVQ adds
> additional state that would make the series bigger and still had some
> controversy on previous RFC, so let's split it.
>
> Successfully tested with vdpa_sim_net with patch [1] applied and with the qemu
> emulated device with vp_vdpa with some restrictions:
> * No CVQ. No feature that didn't work with SVQ previously (packed, ...)
> * VIRTIO_RING_F_STATE patches implementing [2].
> * Expose _F_SUSPEND, but ignore it and suspend on ring state fetch like
>   DPDK.
>
> Previous versions were tested by many vendors. Not carrying Tested-by because
> of code changes, so re-testing would be appreciated.
>
> Comments are welcome.
>
> v4:
> - Recover used_idx from guest's vring if device cannot suspend.
> - Fix starting device in the middle of a migration.  Removed some
>   duplication in setting / clearing enable_shadow_vqs and shadow_data
>   members of vhost_vdpa.
> - Fix (again) "Check for SUSPEND in vhost_dev.backend_cap, as
>   .backend_features is empty at the check moment.". It was reverted by
>   mistake in v3.
> - Fix memory leak of iova tree.
> - Properly rewind SVQ as in flight descriptors were still being accounted
>   in vq base.
> - Expand documentation.
>
> v3:
> - Start datapatch in SVQ in device started while migrating.
> - Properly register migration blockers if device present unsupported features.
> - Fix race condition because of not stopping the SVQ until device cleanup.
> - Explain purpose of iova tree in the first patch message.
> - s/dynamycally/dynamically/ in cover letter.
> - at lore.kernel.org/qemu-devel/20230215173850.298832-14-eperezma@redhat.com
>
> v2:
> - Check for SUSPEND in vhost_dev.backend_cap, as .backend_features is empty at
>   the check moment.
> - at https://lore.kernel.org/all/20230208094253.702672-12-eperezma@redhat.com/T/
>
> v1:
> - Omit all code working with CVQ and block migration if the device supports
>   CVQ.
> - Remove spurious kick.
> - Move all possible checks for migration to vhost-vdpa instead of the net
>   backend. Move them to init code from start code.
> - Suspend on vhost_vdpa_dev_start(false) instead of in vhost-vdpa net backend.
> - Properly split suspend after geting base and adding of status_reset patches.
> - Add possible TODOs to points where this series can improve in the future.
> - Check the state of migration using migration_in_setup and
>   migration_has_failed instead of checking all the possible migration status in
>   a switch.
> - Add TODO with possible low hand fruit using RESUME ops.
> - Always offer _F_LOG from virtio/vhost-vdpa and let migration blockers do
>   their thing instead of adding a variable.
> - RFC v2 at https://lists.gnu.org/archive/html/qemu-devel/2023-01/msg02574.html
>
> RFC v2:
> - Use a migration listener instead of a memory listener to know when
>   the migration starts.
> - Add stuff not picked with ASID patches, like enable rings after
>   driver_ok
> - Add rewinding on the migration src, not in dst
> - RFC v1 at https://lists.gnu.org/archive/html/qemu-devel/2022-08/msg01664.html
>
> [1] https://lore.kernel.org/lkml/20230203142501.300125-1-eperezma@redhat.com/T/
> [2] https://lists.oasis-open.org/archives/virtio-comment/202103/msg00036.html
>
> Eugenio Pérez (15):
>   vdpa net: move iova tree creation from init to start
>   vdpa: Remember last call fd set
>   vdpa: stop svq at vhost_vdpa_dev_start(false)
>   vdpa: Negotiate _F_SUSPEND feature
>   vdpa: move vhost reset after get vring base
>   vdpa: add vhost_vdpa->suspended parameter
>   vdpa: add vhost_vdpa_suspend
>   vdpa: rewind at get_base, not set_base
>   vdpa: add vdpa net migration state notifier
>   vdpa: disable RAM block discard only for the first device
>   vdpa net: block migration if the device has CVQ
>   vdpa: block migration if device has unsupported features
>   vdpa: block migration if SVQ does not admit a feature
>   vdpa net: allow VHOST_F_LOG_ALL
>   vdpa: return VHOST_F_LOG_ALL in vhost-vdpa devices
>
>  include/hw/virtio/vhost-backend.h  |   4 +
>  include/hw/virtio/vhost-vdpa.h     |   3 +
>  hw/virtio/vhost-shadow-virtqueue.c |   8 +-
>  hw/virtio/vhost-vdpa.c             | 128 +++++++++++++------
>  hw/virtio/vhost.c                  |   3 +
>  net/vhost-vdpa.c                   | 198 ++++++++++++++++++++++++-----
>  hw/virtio/trace-events             |   1 +
>  7 files changed, 273 insertions(+), 72 deletions(-)
>
> --

The migration works with SolidNET DPU.

Tested-by: Alvaro Karsz <alvaro.karsz@solid-run.com>