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[RFC v3 00/29] vDPA software assisted live migration

From: Eugenio Pérez
Subject: [RFC v3 00/29] vDPA software assisted live migration
Date: Wed, 19 May 2021 18:28:34 +0200

This series enable shadow virtqueue for vhost-vdpa devices. This is a
new method of vhost devices migration: Instead of relay on vhost
device's dirty logging capability, SW assisted LM intercepts dataplane,
forwarding the descriptors between VM and device. Is intended for vDPA
devices with no dirty memory tracking capabilities.

In this migration mode, qemu offers a new vring to the device to
read and write into, and disable vhost notifiers, processing guest and
vhost notifications in qemu. On used buffer relay, qemu will mark the
dirty memory as with plain virtio-net devices. This way, devices does
not need to have dirty page logging capability.

This series is a POC doing SW LM for vhost-net and vhost-vdpa devices.
The former already have dirty page logging capabilities, but it is both
easier to test and uses different code paths in qemu.

For qemu to use shadow virtqueues the vhost-net devices need to be
* With IOMMU (iommu_platform=on,ats=on)
* Without event_idx (event_idx=off)

And shadow virtqueue needs to be enabled for them with QMP command

{ "execute": "x-vhost-enable-shadow-vq",
      "arguments": { "name": "dev0", "enable": true } }

The series includes some commits to delete in the final version. One
of them is the one that adds vhost_kernel_vring_pause to vhost kernel
devices. This is only intended to work with vhost-net devices, as a way
to test the solution, so don't use any other vhost kernel device in the
same test.

The vhost-vdpa devices should work the same way. However, vp_vdpa is
not working properly with intel iommu unmapping, so this series add two
extra commits to allow testing the solution enable SVQ mode from the
device start and forbidding any other vhost-vdpa memory mapping. The
causes of this are still under debugging.

For testing vhost-vdpa devices vp_vdpa device has been used with nested
virtualization, using a qemu virtio-net device in L0. To be able to
stop and reset status, features in RFC status has been implemented in
commits 5 and 6. After that, virtio-net driver in L0 guest is replaced
by vp_vdpa driver, and a nested qemu instance is launched using it.

This vp_vdpa driver needs to be also modified to support the RFCs,
mainly allowing it to removing the _S_STOPPED status flag and
implementing actual vp_vdpa_set_vq_state and vp_vdpa_get_vq_state

Just the notification forwarding (with no descriptor relay) can be
achieved with patches 7 and 8, and starting SVQ. Previous commits
are cleanup ones and declaration of QMP command.

Commit 17 introduces the buffer forwarding. Previous one are for
preparations again, and laters are for enabling some obvious
optimizations. However, it needs the vdpa device to be able to map
every IOVA space, and some vDPA devices are not able to do so. Checking
of this is added in previous commits.

Later commits allow vhost and shadow virtqueue to track and translate
between qemu virtual addresses and a restricted iommu range. At the
moment is not able to delete old translations, limit maximum range
it can translate, nor vhost add new memory regions from the moment
SVQ is enabled, but is somehow straightforward to add these.

This is a big series, so the idea is to send it in logical chunks once
all comments have been collected. As a first complete usecase, a SVQ
mode with no possibility of going back to regular mode would cover a
first usecase, and this RFC already have all the ingredients but
internal memory tracking.

It is based on the ideas of DPDK SW assisted LM, in the series of
DPDK's https://patchwork.dpdk.org/cover/48370/ . However, these does
not map the shadow vq in guest's VA, but in qemu's.

Comments are welcome!

* Event, indirect, packed, and others features of virtio - Waiting for
  confirmation of the big picture.
* vDPA devices: Grow IOVA tree to track new or deleted memory. Cap
  IOVA limit in tree so it cannot grow forever.
* To sepparate buffers forwarding in its own AIO context, so we can
  throw more threads to that task and we don't need to stop the main
  event loop.
* IOMMU optimizations, so bacthing and bigger chunks of IOVA can be
  sent to device.
* Automatic kick-in on live-migration.
* Proper documentation.


Changes from v2 RFC:
  * Adding vhost-vdpa devices support
  * Fixed some memory leaks pointed by different comments

Changes from v1 RFC:
  * Use QMP instead of migration to start SVQ mode.
  * Only accepting IOMMU devices, closer behavior with target devices
  * Fix invalid masking/unmasking of vhost call fd.
  * Use of proper methods for synchronization.
  * No need to modify VirtIO device code, all of the changes are
    contained in vhost code.
  * Delete superfluous code.
  * An intermediate RFC was sent with only the notifications forwarding
    changes. It can be seen in
  * v1 at

Eugenio Pérez (29):
  virtio: Add virtio_queue_is_host_notifier_enabled
  vhost: Save masked_notifier state
  vhost: Add VhostShadowVirtqueue
  vhost: Add x-vhost-enable-shadow-vq qmp
  vhost: Route guest->host notification through shadow virtqueue
  vhost: Route host->guest notification through shadow virtqueue
  vhost: Avoid re-set masked notifier in shadow vq
  virtio: Add vhost_shadow_vq_get_vring_addr
  vhost: Add vhost_vring_pause operation
  vhost: add vhost_kernel_vring_pause
  vhost: Add vhost_get_iova_range operation
  vhost: add vhost_has_limited_iova_range
  vhost: Add enable_custom_iommu to VhostOps
  vhost-vdpa: Add vhost_vdpa_enable_custom_iommu
  vhost: Shadow virtqueue buffers forwarding
  vhost: Use vhost_enable_custom_iommu to unmap everything if available
  vhost: Check for device VRING_USED_F_NO_NOTIFY at shadow virtqueue
  vhost: Use VRING_AVAIL_F_NO_INTERRUPT at device call on shadow
  vhost: Add VhostIOVATree
  vhost: Add iova_rev_maps_find_iova to IOVAReverseMaps
  vhost: Use a tree to store memory mappings
  vhost: Add iova_rev_maps_alloc
  vhost: Add custom IOTLB translations to SVQ
  vhost: Map in vdpa-dev
  vhost-vdpa: Implement vhost_vdpa_vring_pause operation
  vhost-vdpa: never map with vDPA listener
  vhost: Start vhost-vdpa SVQ directly

 qapi/net.json                                 |  22 +
 hw/virtio/vhost-iova-tree.h                   |  61 ++
 hw/virtio/vhost-shadow-virtqueue.h            |  38 ++
 hw/virtio/virtio-pci.h                        |   1 +
 include/hw/virtio/vhost-backend.h             |  16 +
 include/hw/virtio/vhost-vdpa.h                |   2 +-
 include/hw/virtio/vhost.h                     |  14 +
 include/hw/virtio/virtio.h                    |   5 +-
 .../standard-headers/linux/virtio_config.h    |   5 +
 include/standard-headers/linux/virtio_pci.h   |   2 +
 hw/net/virtio-net.c                           |   4 +-
 hw/virtio/vhost-backend.c                     |  42 ++
 hw/virtio/vhost-iova-tree.c                   | 283 ++++++++
 hw/virtio/vhost-shadow-virtqueue.c            | 643 ++++++++++++++++++
 hw/virtio/vhost-vdpa.c                        |  73 +-
 hw/virtio/vhost.c                             | 459 ++++++++++++-
 hw/virtio/virtio-pci.c                        |   9 +
 hw/virtio/virtio.c                            |   5 +
 hw/virtio/meson.build                         |   2 +-
 hw/virtio/trace-events                        |   1 +
 20 files changed, 1663 insertions(+), 24 deletions(-)
 create mode 100644 hw/virtio/vhost-iova-tree.h
 create mode 100644 hw/virtio/vhost-shadow-virtqueue.h
 create mode 100644 hw/virtio/vhost-iova-tree.c
 create mode 100644 hw/virtio/vhost-shadow-virtqueue.c


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