Re: [PATCH v4 0/8] vhost-user: Back-end state migration

[Stefan Hajnoczi]

On Wed, Oct 04, 2023 at 02:58:56PM +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
> 
> v3:
> https://lists.nongnu.org/archive/html/qemu-devel/2023-09/msg03750.html
> 
> 
> Based-on: <20231004014532.1228637-1-stefanha@redhat.com>
>           ([PATCH v2 0/3] vhost: clean up device reset)
> 
> 
> Hi,
> 
> This v4 includes largely unchanged patches from v3.  The main
> addition/change is what came out of the discussion between Stefan and me
> around how to proceed without SUSPEND/RESUME, which is that this series
> is now based on his reset fix, and it includes more documentation
> changes.

This looks good. I posted some minor comments on the new patches.

Stefan

> 
> Changes in detail:
> 
> - Patch 1: Fall-out from the reset fix: Currently, the status byte is
>   effectively unused (qemu only uses it for resetting, which all
>   back-ends ignore; DPDK uses it to announce potential feature
>   negotiation failure, which qemu ignores).  It is also not defined what
>   exactly front-end or back-end should do with this byte, except
>   pointing at the virtio spec, which however naturally does not say how
>   this integrates with vhost-user’s RESET_DEVICE or [GS]ET_FEATURES.
>   Furthermore, there does not seem to be a use for this; we have
>   RESET_DEVICE for resetting, and we have [GS]ET_FEATURES (and
>   REPLY_ACK, which can be used on SET_FEATURES) for feature
>   negotation.
>   Therefore, deprecate the status byte, pointing to those other commands
>   instead.
> 
> - Patch 2: Patch 4 defines a suspended state for the whole back-end if
>   all vrings are stopped.  I think this should be mentioned in
>   GET_VRING_BASE, but upon trying to add it, I found that it does not
>   even mention that it stops the vring (mentioned only in the Ring
>   States section), and remembered that the whole description of both
>   GET_VRING_BASE and SET_VRING_BASE really was not helpful when trying
>   to implement a vhost-user back-end.  Took the opportunity to overhaul
>   both.
> 
> - Patch 3: This one’s from v3, but quite heavily modified.  Stefan
>   suggested consistently defining the started/stopped and
>   enabled/disabled states to be independent, and indeed doing so
>   simplifies a whole lot of stuff.  Specifically, it makes the magic
>   “enabled/disabled when started” go away.  Basically, I found this
>   change alone is enough to remove the confusion I had with the existing
>   documentation.
> 
> - Patch 4: As suggested by Stefan, just define a suspended state without
>   introducing SUSPEND.  vDPA needs SUSPEND because its GET_VRING_BASE
>   does not stop the vring, but vhost-user’s does, so we can define the
>   suspended state to be when all vrings are stopped.
> 
> - Patch 5: Reference the suspended state.
> 
> - Patches 6 through 8: Unmodified, except for them being rebase on
>   Stefan’s series.
> 
> 
> Hanna Czenczek (8):
>   vhost-user.rst: Deprecate [GS]ET_STATUS
>   vhost-user.rst: Improve [GS]ET_VRING_BASE doc
>   vhost-user.rst: Clarify enabling/disabling vrings
>   vhost-user.rst: Introduce suspended state
>   vhost-user.rst: Migrating back-end-internal state
>   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       | 318 +++++++++++++++++++++++++++---
>  include/hw/virtio/vhost-backend.h |  24 +++
>  include/hw/virtio/vhost.h         | 113 +++++++++++
>  hw/virtio/vhost-user-fs.c         | 101 +++++++++-
>  hw/virtio/vhost-user.c            | 148 ++++++++++++++
>  hw/virtio/vhost.c                 | 241 ++++++++++++++++++++++
>  6 files changed, 917 insertions(+), 28 deletions(-)
> 
> -- 
> 2.41.0
>

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