Re: [virtio-comment] [RFC PATCH 0/5] virtio: introduce SUSPEND bit and vq state

[Jason Wang <jasowang@redhat.com>]

On Tue, Aug 15, 2023 at 12:28âAM Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> On Tue, Aug 15, 2023 at 03:28:59AM +0800, Zhu Lingshan wrote:
> > This seires introduces
> > 1)a new SUSPEND bit in the device status
> > Which is used to suspend the device, so that the device states
> > and virtqueue states are stablized.
> >
> > 2)virtqueue state and accessors, to get and set last_avail_idx
> > and last_used_idx of virtqueues.
> >
> > The main usecase of these new facilities is Live Migration.
> >
> > Furture work: dirty page tracking and in-flight descriptors.
> >
> > This is RFC, this series carries on Jason and Eugenio's pervious work.
> >
> > Any comments are welcome.
>
> In order to support stateful devices like virtiofs, virtio-gpu, or
> virtio-crypto it would be necessary to add device state save/load
> functionality too.

+1 and before save/load, we need to define those states first.

E.g there are patches that try to fix the GPU suspend/hibernation
which probably requires those facilities as well.

>
> Even virtio-net needs device state save/load functionality if the VMM is
> not intercepting the stateful controlq.

Yes, this could be done via trap and emulation.

>
> In-flight descriptors can be included in the device state.
>

Probably.

> In fact, the only reason why I think VIRTIO_F_QUEUE_STATE makes sense is
> for easy integration into existing vhost software stacks. Otherwise we
> should just define a device state save/load interface along with
> standard device state serialization for devices where migration
> interoperability is possible. VIRTIO_F_QUEUE_STATE is a small subset of
> device state save/load.

For simple devices like virtio-net, queue state should be sufficient
for hardware.

>
> I think the convenience of integrating into existing vhost software
> stacks is worth the duplication, but I wanted to mention that this
> interface will not be enough to live migrate all device types and we'll
> need something more in the future.

Yes, the patch is a start for simple stateless devices.

Thanks

>
> Stefan
>
> >
> > Zhu Lingshan (5):
> >   virtio: introduce SUSPEND bit in device status
> >   virtio: introduce vq state as basic facility
> >   virtio: The actions by the device upon SUSPEND
> >   virtqueue: constraints for virtqueue state
> >   virtio-pci: implement VIRTIO_F_QUEUE_STATE
> >
> >  content.tex       | 120 ++++++++++++++++++++++++++++++++++++++++++++++
> >  transport-pci.tex |  15 ++++++
> >  2 files changed, 135 insertions(+)
> >
> > --
> > 2.35.3
> >
> >
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