Re: [PATCH v3 0/5] vhost-user: Back-end state migration

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Sep 15, 2023 at 12:25:25PM +0200, Hanna Czenczek wrote:
> RFC:
> https://lists.nongnu.org/archive/html/qemu-devel/2023-03/msg04263.html
> 
> v1:
> https://lists.nongnu.org/archive/html/qemu-devel/2023-04/msg01575.html
> 
> v2:
> https://lists.nongnu.org/archive/html/qemu-devel/2023-07/msg02604.html
> 
> Hi,
> 
> I’ve decided not to work on vhost-user SUSPEND/RESUME for now – it is
> not technically required for virtio-fs migration, which is the actual
> priority for me now.  While we do want to have SUSPEND/RESUME at some
> point, the only practically existing reason for it is to be able to
> implement vhost-level resetting in virtiofsd, but that is not related to
> migration.

QEMU sends VHOST_USER_SET_STATUS 0 in vhost_dev_stop(). Are you assuming
that virtiofs back-ends do not reset the device upon receiving this
message?

> So one of the changes in v3 is that it no longer depends on the
> vhost-user SUSPEND/RESUME series, and describes the migration protocol
> without the device being suspended at any point, but merely that the
> vrings are stopped.
> 
> Other changes include:
> 
> - Patch 1:
>   - Rephrased a lot
>   - Added a description for the VHOST_USER_SET_DEVICE_STATE_FD
>     parameters
>   - Renamed VHOST_USER_PROTOCOL_F_MIGRATORY_STATE to
>     VHOST_USER_PROTOCOL_F_DEVICE_STATE
>   - enum variants changed in value due to dropping the SUSPEND/RESUME
>     dependency
> 
> - Patch 2:
>   - Pulled in, was a stand-alone patch before
>   - Dropped a sentence about ring state before feature negotiations, as
>     the rings are not to be used during that period anyway
>   - Bit of rephrasing
> 
> - Patch 3:
>   - Renamed “migratory state” to “device state”
>   - enum variants changed in value due to dropping the SUSPEND/RESUME
>     dependency
> 
> - Patch 4:
>   - Changed `f` to @f (referencing parameter “f”) in comments
>   - Use g_autofree for the transfer buffer
>   - Note SUSPEND state as a future feature, not currently existing
>   - Wrap read() and write() in RETRY_ON_EINTR()
> 
> - Patch 5:
>   - Renamed “migratory state” to “device state”
>   - (kept R-b still)
> 
> 
> Hanna Czenczek (5):
>   vhost-user.rst: Migrating back-end-internal state
>   vhost-user.rst: Clarify enabling/disabling vrings
>   vhost-user: Interface for migration state transfer
>   vhost: Add high-level state save/load functions
>   vhost-user-fs: Implement internal migration
> 
>  docs/interop/vhost-user.rst       | 188 ++++++++++++++++++++++-
>  include/hw/virtio/vhost-backend.h |  24 +++
>  include/hw/virtio/vhost.h         | 114 ++++++++++++++
>  hw/virtio/vhost-user-fs.c         | 101 ++++++++++++-
>  hw/virtio/vhost-user.c            | 148 ++++++++++++++++++
>  hw/virtio/vhost.c                 | 241 ++++++++++++++++++++++++++++++
>  6 files changed, 810 insertions(+), 6 deletions(-)
> 
> -- 
> 2.41.0
> 

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