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

["Zhu, Lingshan" <lingshan.zhu@intel.com>]

On 9/14/2023 7:37 PM, Michael S. Tsirkin wrote:
> On Wed, Sep 06, 2023 at 04:16:32PM +0800, Zhu Lingshan wrote:
> > This series 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 stabilized.
> >
> > 2)virtqueue state and its accessor, to get and set last_avail_idx
> > and last_used_idx of virtqueues.
> >
> > The main usecase of these new facilities is Live Migration.
> >
> > Future work: dirty page tracking and in-flight descriptors.
> >
> > This series addresses many comments from Jason, Stefan and Eugenio
> > from RFC series.
>
> after going over this in detail, it is like I worried: this
> tries to do too much through a single register and
> the ownership is muddied significantly.
Not sure about what ownership, device usually STOPPED after
guest freezes, so the hypervisor owns the device status
and LM facilities at that moment.
> I feel a separate capability for suspend/resume that would
> be independent of device status would be preferable.
The implementation of the live migration basic facilities are transport 
specific, for PCI:
1)Dirty page tracking will have its own capability
2)In-flight descriptors tracker will have its own capability
3)vq states stored in common config space

Only SUSPEND is implemented in the device status, and this is a valid 
device status.
There are already 6 device status bits, and IMHO this series 
implementing SUSPEND does not
introduce more complexities.
> > Zhu Lingshan (5):
> >    virtio: introduce vq state as basic facility
> >    virtio: introduce SUSPEND bit in device status
> >    virtqueue: constraints for virtqueue state
> >    virtqueue: ignore resetting vqs when SUSPEND
> >    virtio-pci: implement VIRTIO_F_QUEUE_STATE
> >
> >   content.tex       | 118 ++++++++++++++++++++++++++++++++++++++++++++++
> >   transport-pci.tex |  18 +++++++
> >   2 files changed, 136 insertions(+)
> >
> > --
> > 2.35.3
>
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