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Re: [RFC PATCH 00/27] vDPA software assisted live migration

From: Jason Wang
Subject: Re: [RFC PATCH 00/27] vDPA software assisted live migration
Date: Wed, 25 Nov 2020 15:08:32 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 2020/11/21 上午2:50, Eugenio Pérez wrote:
This series enable vDPA software assisted live migration for vhost-net
devices. This is a new method of vhost devices migration: Instead of
relay on vDPA device's dirty logging capability, SW assisted LM
intercepts dataplane, forwarding the descriptors between VM and device.

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. None of the changes have actual
effect with current devices until last two patches (26 and 27) are
applied, but they can be rebased on top of any other. These checks the
device to meet all requirements, and disable vhost-net devices logging
so migration goes through SW LM. This last patch is not meant to be
applied in the final revision, it is in the series just for testing

For use SW assisted LM these vhost-net devices need to be instantiated:
* With IOMMU (iommu_platform=on,ats=on)
* Without event_idx (event_idx=off)

So a question is at what level do we want to implement qemu assisted live migration. To me it could be done at two levels:

1) generic vhost level which makes it work for both vhost-net/vhost-user and vhost-vDPA
2) a specific type of vhost

To me, having a generic one looks better but it would be much more complicated. So what I read from this series is it was a vhost kernel specific software assisted live migration which is a good start. Actually it may even have real use case, e.g it can save dirty bitmaps for guest with large memory. But we need to address the above limitations first.

So I would like to know what's the reason for mandating iommu platform and ats? And I think we need to fix case of event idx support.

Just the notification forwarding (with no descriptor relay) can be
achieved with patches 7 and 9, and starting migration. Partial applies
between 13 and 24 will not work while migrating on source, and patch
25 is needed for the destination to resume network activity.

It is based on the ideas of DPDK SW assisted LM, in the series of

Actually we're better than that since there's no need the trick like hardcoded IOVA for mediated(shadow) virtqueue.

DPDK's https://patchwork.dpdk.org/cover/48370/ .

I notice that you do GPA->VA translations and try to establish a VA->VA (use VA as IOVA) mapping via device IOTLB. This shortcut should work for vhost-kernel/user but not vhost-vDPA. The reason is that there's no guarantee that the whole 64bit address range could be used as IOVA. One example is that for hardware IOMMU like intel, it usually has 47 or 52 bits of address width.

So we probably need an IOVA allocator that can make sure the IOVA is not overlapped and fit for [1]. We can probably build the IOVA for guest VA via memory listeners. Then we have

1) IOVA for GPA
2) IOVA for shadow VQ

And advertise IOVA to VA mapping to vhost.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1b48dc03e575a872404f33b04cd237953c5d7498

Comments are welcome.


Eugenio Pérez (27):
   vhost: Add vhost_dev_can_log
   vhost: Add device callback in vhost_migration_log
   vhost: Move log resize/put to vhost_dev_set_log
   vhost: add vhost_kernel_set_vring_enable
   vhost: Add hdev->dev.sw_lm_vq_handler
   virtio: Add virtio_queue_get_used_notify_split
   vhost: Route guest->host notification through qemu
   vhost: Add a flag for software assisted Live Migration
   vhost: Route host->guest notification through qemu
   vhost: Allocate shadow vring
   virtio: const-ify all virtio_tswap* functions
   virtio: Add virtio_queue_full
   vhost: Send buffers to device
   virtio: Remove virtio_queue_get_used_notify_split
   vhost: Do not invalidate signalled used
   virtio: Expose virtqueue_alloc_element
   vhost: add vhost_vring_set_notification_rcu
   vhost: add vhost_vring_poll_rcu
   vhost: add vhost_vring_get_buf_rcu
   vhost: Return used buffers
   vhost: Add vhost_virtqueue_memory_unmap
   vhost: Add vhost_virtqueue_memory_map
   vhost: unmap qemu's shadow virtqueues on sw live migration
   vhost: iommu changes
   vhost: Do not commit vhost used idx on vhost_virtqueue_stop
   vhost: Add vhost_hdev_can_sw_lm
   vhost: forbid vhost devices logging

  hw/virtio/vhost-sw-lm-ring.h      |  39 +++
  include/hw/virtio/vhost.h         |   5 +
  include/hw/virtio/virtio-access.h |   8 +-
  include/hw/virtio/virtio.h        |   4 +
  hw/net/virtio-net.c               |  39 ++-
  hw/virtio/vhost-backend.c         |  29 ++
  hw/virtio/vhost-sw-lm-ring.c      | 268 +++++++++++++++++++
  hw/virtio/vhost.c                 | 431 +++++++++++++++++++++++++-----
  hw/virtio/virtio.c                |  18 +-
  hw/virtio/meson.build             |   2 +-
  10 files changed, 758 insertions(+), 85 deletions(-)
  create mode 100644 hw/virtio/vhost-sw-lm-ring.h
  create mode 100644 hw/virtio/vhost-sw-lm-ring.c

So this looks like a pretty huge patchset which I'm trying to think of ways to split. An idea is to do this is two steps

1) implement a shadow virtqueue mode for vhost first (w/o live migration). Then we can test descriptors relay, IOVA allocating, etc.
2) add live migration support on top

And it looks to me it's better to split the shadow virtqueue (virtio driver part) into an independent file. And use generic name (w/o "shadow") in order to be reused by other use cases as well.


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