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

[Jason Wang <jasowang@redhat.com>]

On 2020/11/25 下午8:03, Eugenio Perez Martin wrote:
> On Wed, Nov 25, 2020 at 8:09 AM Jason Wang <jasowang@redhat.com> wrote:
>>
>> 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
>>> purposes.
>>>
>>> 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.
>>
> There is no specific reason for mandating iommu & ats, it was just
> started that way.
>
> I will extend the patch to support those cases too.
>
>>> 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
>>
> Got it, will control it too.
>
> Maybe for vhost-net we could directly send iotlb miss for [0,~0ULL].


It works but it means vhost-net needs some special care. To me a generic 
IOVA allocator looks better.


>
>>> Comments are welcome.
>>>
>>> Thanks!
>>>
>>> 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.
> How would that mode be activated if it is not tied to live migration?
> New backend/command line switch?


Either a new cli option or even a qmp command can work.


>
> Maybe it is better to also start with no iommu & ats support and add it on top.


Yes.


>
>> 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.
>>
> I think the same.
>
> Thanks!
>
>> Thoughts?
>>
>