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Re: [RFC v2 00/13] vDPA software assisted live migration

From: Jason Wang
Subject: Re: [RFC v2 00/13] vDPA software assisted live migration
Date: Tue, 16 Mar 2021 16:28:20 +0800
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.16; rv:78.0) Gecko/20100101 Thunderbird/78.8.1

在 2021/3/16 上午3:48, Eugenio Pérez 写道:
This series enable shadow virtqueue for vhost-net 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 logging, but this address the basic platform to build
that support on.

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 devices, which already
have dirty page logging capabilities. For qemu to use shadow virtqueues
these vhost-net devices need to be instantiated:
* 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": "virtio-net",
     "enable": false

Just the notification forwarding (with no descriptor relay) can be
achieved with patches 5 and 6, and starting SVQ. Previous commits
are cleanup ones and declaration of QMP command.

Commit 11 introduces the buffer forwarding. Previous one are for
preparations again, and laters are for enabling some obvious

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! Especially on:
* Different/improved way of synchronization, particularly on the race
   of masking.

* Event, indirect, packed, and others features of virtio - Waiting for
   confirmation of the big picture.

So two things in my mind after reviewing the seires:

1) which layer should we implement the shadow virtqueue. E.g if you want to do that at virtio level, you need to deal with a lot of synchronizations. I prefer to do it in vhost-vDPA.
2) Using VA as IOVA which can not work for vhost-vDPA

* vDPA devices:

So I think we can start from a vhost-vDPA specific shadow virtqueue first, then extending it to be a general one which might be much easier.

Developing solutions for tracking the available IOVA
   space for all devices.

For vhost-net, you can assume that [0, ULLONG_MAX] is valid so you can simply use VA as IOVA.

Small POC available, skipping the get/set
   status (since vDPA does not support it) and just allocating more and
   more IOVA addresses in a hardcoded range available for the device.

I'm not sure this can work but you need make sure that range can fit the size of the all memory regions and need to deal with memory region add and del.

I guess you probably need a full functional tree based IOVA allocator.


* 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 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 (13):
   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
   virtio: Add virtio_queue_full
   vhost: add vhost_kernel_set_vring_enable
   vhost: Shadow virtqueue buffers forwarding
   vhost: Check for device VRING_USED_F_NO_NOTIFY at shadow virtqueue
   vhost: Use VRING_AVAIL_F_NO_INTERRUPT at device call on shadow

  qapi/net.json                      |  22 ++
  hw/virtio/vhost-shadow-virtqueue.h |  36 ++
  include/hw/virtio/vhost.h          |   6 +
  include/hw/virtio/virtio.h         |   3 +
  hw/virtio/vhost-backend.c          |  29 ++
  hw/virtio/vhost-shadow-virtqueue.c | 551 +++++++++++++++++++++++++++++
  hw/virtio/vhost.c                  | 283 +++++++++++++++
  hw/virtio/virtio.c                 |  23 +-
  hw/virtio/meson.build              |   2 +-
  9 files changed, 952 insertions(+), 3 deletions(-)
  create mode 100644 hw/virtio/vhost-shadow-virtqueue.h
  create mode 100644 hw/virtio/vhost-shadow-virtqueue.c

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