RE: [virtio-dev] Re: [virtio-comment] Re: [VIRTIO PCI PATCH v5 1/1] transport-pci: Add freeze_mode to virtio_pci_common_cfg

[Parav Pandit]

> From: Zhu, Lingshan <lingshan.zhu@xxxxxxxxx>
> Sent: Wednesday, September 20, 2023 12:58 PM
> 
> On 9/20/2023 3:10 PM, Parav Pandit wrote:
> >> From: Zhu, Lingshan <lingshan.zhu@xxxxxxxxx>
> >> Sent: Wednesday, September 20, 2023 12:37 PM
> >>> The problem to overcome in [1] is, resume operation needs to be
> >>> synchronous
> >> as it involves large part of context to resume back, and hence just
> >> asynchronously setting DRIVER_OK is not enough.
> >>> The sw must verify back that device has resumed the operation and
> >>> ready to
> >> answer requests.
> >> this is not live migration, all device status and other information
> >> still stay in the device, no need to "resume" context, just resume running.
> >>
> > I am aware that it is not live migration. :)
> >
> > "Just resuming" involves lot of device setup task. The device implementation
> does not know for how long a device is suspended.
> > So for example, a VM is suspended for 6 hours, hence the device context
> could be saved in a slow disk.
> > Hence, when the resume is done, it needs to setup things again and driver got
> to verify before accessing more from the device.
> The restore procedures should perform by the hypervisor and done before set
> DRIVER_OK and wake up the guest.
Which is the signal to trigger the restore? Which is the trigger in physical device when there is no hypervisor?

In my view, setting the DRIVER_OK is the signal regardless of hypervisor or physical device.
Hence the re-read is must.

> And the hypervisor/driver needs to check the device status by re-reading.
> >
> >> Like resume from a failed LM.
> >>> This is slightly different flow than setting the DRIVER_OK for the
> >>> first time
> >> device initialization sequence as it does not involve large restoration.
> >>> So, to merge two ideas, instead of doing DRIVER_OK to resume, the
> >>> driver
> >> should clear the SUSPEND bit and verify that it is out of SUSPEND.
> >>> Because driver is still in _OK_ driving the device flipping the SUSPEND bit.
> >> Please read the spec, it says:
> >> The driver MUST NOT clear a device status bit
> >>
> > Yes, this is why either DRIER_OK validation by the driver is needed or Jiqian's
> synchronous new register..
> so re-read
Yes. re-read until set, Thanks.