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

[Eugenio Perez Martin]

On Wed, Nov 25, 2020 at 1:03 PM Eugenio Perez Martin
<eperezma@redhat.com> 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].
>

Sorry, this was intended to be a question :).

Given a vhost-* device IOVA usable range, is ok to expose all of qemu
overlapping VA to it? With the iotlb miss, for example. Would it be
acceptable from a security point of view? The device would have access
to all qemu VA, but on the other hand devices like vhost-net already
have it.

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