Re: [PATCH v2 00/13] Dynamycally switch to vhost shadow virtqueues at vdpa net migration

[Eugenio Perez Martin]

On Fri, Feb 10, 2023 at 1:58 PM Gautam Dawar <gdawar@amd.com> wrote:
>
> Hi Eugenio,
>
> I've tested this patch series on Xilinx/AMD SN1022 device without
> control vq and VM Live Migration between two hosts worked fine.
>
> Tested-by: Gautam Dawar <gautam.dawar@amd.com>
>

Thanks for the testing!

>
> Here is some minor feedback:
>
> Pls fix the typo (Dynamycally -> Dynamically) in the Subject.
>
> On 2/8/23 15:12, Eugenio Pérez wrote:
> > CAUTION: This message has originated from an External Source. Please use 
> > proper judgment and caution when opening attachments, clicking links, or 
> > responding to this email.
> >
> >
> > It's possible to migrate vdpa net devices if they are shadowed from the
> >
> > start.  But to always shadow the dataplane is to effectively break its host
> >
> > passthrough, so its not convenient in vDPA scenarios.
> I believe you meant efficient instead of convenient.
> >
> >
> >
> > This series enables dynamically switching to shadow mode only at
> >
> > migration time.  This allows full data virtqueues passthrough all the
> >
> > time qemu is not migrating.
> >
> >
> >
> > In this series only net devices with no CVQ are migratable.  CVQ adds
> >
> > additional state that would make the series bigger and still had some
> >
> > controversy on previous RFC, so let's split it.
> >
> >
> >
> > The first patch delays the creation of the iova tree until it is really 
> > needed,
> >
> > and makes it easier to dynamically move from and to SVQ mode.
> It would help adding some detail on the iova tree being referred to here.
> >
> >
> >
> > Next patches from 02 to 05 handle the suspending and getting of vq state 
> > (base)
> >
> > of the device at the switch to SVQ mode.  The new _F_SUSPEND feature is
> >
> > negotiated and stop device flow is changed so the state can be fetched 
> > trusting
> >
> > the device will not modify it.
> >
> >
> >
> > Since vhost backend must offer VHOST_F_LOG_ALL to be migratable, last 
> > patches
> >
> > but the last one add the needed migration blockers so vhost-vdpa can offer 
> > it
>
> "last patches but the last one"?
>

I think I solved all of the above in v3, thanks for notifying them!

Would it be possible to test with v3 too?

> Thanks.
>
> >
> > safely.  They also add the handling of this feature.
> >
> >
> >
> > Finally, the last patch makes virtio vhost-vdpa backend to offer
> >
> > VHOST_F_LOG_ALL so qemu migrate the device as long as no other blocker has 
> > been
> >
> > added.
> >
> >
> >
> > Successfully tested with vdpa_sim_net with patch [1] applied and with the 
> > qemu
> >
> > emulated device with vp_vdpa with some restrictions:
> >
> > * No CVQ. No feature that didn't work with SVQ previously (packed, ...)
> >
> > * VIRTIO_RING_F_STATE patches implementing [2].
> >
> > * Expose _F_SUSPEND, but ignore it and suspend on ring state fetch like
> >
> >    DPDK.
> >
> >
> >
> > Comments are welcome.
> >
> >
> >
> > v2:
> >
> > - Check for SUSPEND in vhost_dev.backend_cap, as .backend_features is empty 
> > at
> >
> >    the check moment.
> >
> >
> >
> > v1:
> >
> > - Omit all code working with CVQ and block migration if the device supports
> >
> >    CVQ.
> >
> > - Remove spurious kick.
> Even with the spurious kick, datapath didn't resume at destination VM
> after LM as kick happened before DRIVER_OK. So IMO, it will be required
> that the vdpa parent driver simulates a kick after creating/starting HW
> rings.

Right, it did not solve the issue.

If I'm not wrong all vdpa drivers are moving to that model, checking
for new avail descriptors right after DRIVER_OK. Maybe it is better to
keep this discussion at patch 12/13 on RFC v2?

Thanks!

> >
> > - Move all possible checks for migration to vhost-vdpa instead of the net
> >
> >    backend. Move them to init code from start code.
> >
> > - Suspend on vhost_vdpa_dev_start(false) instead of in vhost-vdpa net 
> > backend.
> >
> > - Properly split suspend after geting base and adding of status_reset 
> > patches.
> >
> > - Add possible TODOs to points where this series can improve in the future.
> >
> > - Check the state of migration using migration_in_setup and
> >
> >    migration_has_failed instead of checking all the possible migration 
> > status in
> >
> >    a switch.
> >
> > - Add TODO with possible low hand fruit using RESUME ops.
> >
> > - Always offer _F_LOG from virtio/vhost-vdpa and let migration blockers do
> >
> >    their thing instead of adding a variable.
> >
> > - RFC v2 at 
> > https://lists.gnu.org/archive/html/qemu-devel/2023-01/msg02574.html
> >
> >
> >
> > RFC v2:
> >
> > - Use a migration listener instead of a memory listener to know when
> >
> >    the migration starts.
> >
> > - Add stuff not picked with ASID patches, like enable rings after
> >
> >    driver_ok
> >
> > - Add rewinding on the migration src, not in dst
> >
> > - RFC v1 at 
> > https://lists.gnu.org/archive/html/qemu-devel/2022-08/msg01664.html
> >
> >
> >
> > [1] 
> > https://lore.kernel.org/lkml/20230203142501.300125-1-eperezma@redhat.com/T/
> >
> > [2] 
> > https://lists.oasis-open.org/archives/virtio-comment/202103/msg00036.html
> >
> >
> >
> > Eugenio Pérez (13):
> >
> >    vdpa net: move iova tree creation from init to start
> >
> >    vdpa: Negotiate _F_SUSPEND feature
> >
> >    vdpa: add vhost_vdpa_suspend
> >
> >    vdpa: move vhost reset after get vring base
> >
> >    vdpa: rewind at get_base, not set_base
> >
> >    vdpa net: allow VHOST_F_LOG_ALL
> >
> >    vdpa: add vdpa net migration state notifier
> >
> >    vdpa: disable RAM block discard only for the first device
> >
> >    vdpa net: block migration if the device has CVQ
> >
> >    vdpa: block migration if device has unsupported features
> >
> >    vdpa: block migration if dev does not have _F_SUSPEND
> >
> >    vdpa: block migration if SVQ does not admit a feature
> >
> >    vdpa: return VHOST_F_LOG_ALL in vhost-vdpa devices
> >
> >
> >
> >   include/hw/virtio/vhost-backend.h |   4 +
> >
> >   hw/virtio/vhost-vdpa.c            | 126 +++++++++++++++-----
> >
> >   hw/virtio/vhost.c                 |   3 +
> >
> >   net/vhost-vdpa.c                  | 192 +++++++++++++++++++++++++-----
> >
> >   hw/virtio/trace-events            |   1 +
> >
> >   5 files changed, 267 insertions(+), 59 deletions(-)
> >
> >
> >
> > --
> >
> > 2.31.1
> >
> >
> >
> >
>