Re: [virtio-comment] [PATCH 5/5] virtio-pci: implement VIRTIO_F_QUEUE_STATE

[Jason Wang <jasowang@redhat.com>]

On Mon, Sep 11, 2023 at 2:47âPM Parav Pandit <parav@nvidia.com> wrote:
>
>
>
> > From: Jason Wang <jasowang@redhat.com>
> > Sent: Monday, September 11, 2023 12:01 PM
> >
> > On Mon, Sep 11, 2023 at 12:12âPM Parav Pandit <parav@nvidia.com> wrote:
> > >
> > > Hi Michael,
> > >
> > > > From: virtio-comment@lists.oasis-open.org
> > > > <virtio-comment@lists.oasis- open.org> On Behalf Of Jason Wang
> > > > Sent: Monday, September 11, 2023 8:31 AM
> > > >
> > > > On Wed, Sep 6, 2023 at 4:33âPM Michael S. Tsirkin <mst@redhat.com>
> > wrote:
> > > > >
> > > > > On Wed, Sep 06, 2023 at 04:16:37PM +0800, Zhu Lingshan wrote:
> > > > > > This patch adds two new le16 fields to common configuration
> > > > > > structure to support VIRTIO_F_QUEUE_STATE in PCI transport layer.
> > > > > >
> > > > > > Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
> > > > >
> > > > >
> > > > > I do not see why this would be pci specific at all.
> > > >
> > > > This is the PCI interface for live migration. The facility is not specific to PCI.
> > > >
> > > > It can choose to reuse the common configuration or not, but the
> > > > semantic is general enough to be used by other transports. We can
> > > > introduce one for MMIO for sure.
> > > >
> > > > >
> > > > > But besides I thought work on live migration will use admin queue.
> > > > > This was explicitly one of the motivators.
> > > >
> > > Please find the proposal that uses administration commands for device
> > migration at [1] for passthrough devices.
> > >
> > > [1]
> > > https://lists.oasis-open.org/archives/virtio-comment/202309/msg00061.h
> > > tml
> >
> > This proposal couples live migration with several requirements, and suffers from
> > the exact issues I've mentioned below.
> >
> It does not.
> Can you please list which one?
>
> > In some cases, it's even worse (coupling with PCI/SR-IOV, second state machine
> > other than the device status).
> >
> There is no state machine in [1].

Isn't the migration modes of "active, stop, freeze" a state machine?

> It is not coupled with PCI/SR-IOV either.
> It supports PCI/SR-IOV transport and in future other transports too when they evolve.
>

For example:

+struct virtio_dev_ctx_pci_vq_cfg {
+ le16 vq_index;
+        le16 queue_size;
+        le16 queue_msix_vector;
+ le64 queue_descÃ
+        le64 queue_driverÃ
+        le64 queue_deviceÃ
+};
+\end{lstlisting}

And does this mean we will have commands for MMIO and other transport?
(Most of the fields except the msix are general enough). And it's just
a partial implementation of the queue related functionality of the
common cfg, so I wonder how it can work.

Thanks