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

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Date:   Wed, 20 Sep 2023 07:10:46 +0000
From:   Parav Pandit <parav@...dia.com>
To:     "Zhu, Lingshan" <lingshan.zhu@...el.com>,
        "Chen, Jiqian" <Jiqian.Chen@....com>,
        "Michael S. Tsirkin" <mst@...hat.com>
CC:     Gerd Hoffmann <kraxel@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        David Airlie <airlied@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>,
        Marc-André Lureau <marcandre.lureau@...il.com>,
        Robert Beckett <bob.beckett@...labora.com>,
        Mikhail Golubev-Ciuchea <Mikhail.Golubev-Ciuchea@...nsynergy.com>,
        "virtio-comment@...ts.oasis-open.org" 
        <virtio-comment@...ts.oasis-open.org>,
        "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
        "qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Roger Pau Monné <roger.pau@...rix.com>,
        "Deucher, Alexander" <Alexander.Deucher@....com>,
        "Koenig, Christian" <Christian.Koenig@....com>,
        "Hildebrand, Stewart" <Stewart.Hildebrand@....com>,
        Xenia Ragiadakou <burzalodowa@...il.com>,
        "Huang, Honglei1" <Honglei1.Huang@....com>,
        "Zhang, Julia" <Julia.Zhang@....com>,
        "Huang, Ray" <Ray.Huang@....com>
Subject: RE: [virtio-dev] Re: [virtio-comment] Re: [VIRTIO PCI PATCH v5 1/1]
 transport-pci: Add freeze_mode to virtio_pci_common_cfg


> From: Zhu, Lingshan <lingshan.zhu@...el.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.
 
> 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..