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

[Zhu, Lingshan]

On 9/20/2023 3:10 PM, Parav Pandit wrote:
> > From: Zhu, Lingshan <lingshan.zhu@intel.com>
> > 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.
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
>