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Subject: Re: [VIRTIO GPU PATCH v3 0/1] Add new feature flag VIRTIO_GPU_F_FREEZE_S3

On 2023/8/2 12:49, Parav Pandit wrote:
>> From: virtio-dev@lists.oasis-open.org <virtio-dev@lists.oasis-open.org> On
>> Behalf Of Chen, Jiqian
>> Sent: Wednesday, August 2, 2023 8:51 AM
>> Hi all,
>> Do you have any other comments on the modification of virtio-gpu S3?
>> Looking forward to your reply and comments.

Hi Parav Pandit,
Thank you for your reply. Let me try to answer your question.

> I am not familiar with the GPU, so a dumb question is, why the S3 state is gpu specific?
S3 state is not gpu specific. I think different virtio devices may have different actions/problems when S3. 
When I do S3 on Xen, I found guest's display can't come back and the root cause is in virtio-gpu backend in QEMU.
So, to solve that problem, I change codes to let guest notify QEMU virtio-gpu's suspend state, and then QEMU will not destroy resources that used for display.
Please see attached kernel and QEMU patch links.
For above reason, Gerd suggest me to add a new feature flag specifically for virtio-gpu, so that guest and host can negotiate whenever to enable above mechanism.

> Can a transport specific suspend state be used and apply to all virtio devices?
Based on my limited knowledge. Different virtio devices have different virtio queues, my modifications let guest to notify QEMU by using virtio-gpu's control queue.
Other virtio devices can't get that notification unless you can traverse all virtio devices and notify one by one, or other global method?
But for now, this patch is to add a new feature flag just used for virtio-gpu.

> And can you please add both the rationale to the commit message?
Sure, I will expand the description of my commit message and add them.

>> On 2023/7/20 20:18, Jiqian Chen wrote:
>>> v3:
>>> Hi all,
>>> Thanks for Gerd Hoffmann's advice. V3 makes below changes:
>>> * Use enum for freeze mode, so this can be extended with more
>>>   modes in the future.
>>> * Rename functions and paratemers with "_S3" postfix.
>>> * Explain in more detail
>>> And latest version on QEMU and Linux kernel side:
>>> 	QEMU: https://lore.kernel.org/qemu-devel/20230720120816.8751-1-
>> Jiqian.Chen@amd.com
>>> 	Kernel:
>>> https://lore.kernel.org/lkml/20230720115805.8206-1-Jiqian.Chen@amd.com
>>> /T/#t
>>> Best regards,
>>> Jiqian Chen.
>>> v2:
>>> link,
>>> https://lists.oasis-open.org/archives/virtio-comment/202307/msg00160.h
>>> tml
>>> Hi all,
>>> Thanks to Gerd Hoffmann for his suggestions. V2 makes below changes:
>>> * Elaborate on the types of resources.
>>> * Add some descriptions for S3 and S4.
>>> v1:
>>> link,
>>> https://lists.oasis-open.org/archives/virtio-comment/202306/msg00595.h
>>> tml
>>> Hi all,
>>> I am working to implement virtgpu S3 function on Xen.
>>> Currently on Xen, if we start a guest through Qemu with enabling
>>> virtgpu, and then suspend and s3resume guest. We can find that the
>>> guest kernel comes back, but the display doesn't. It just shown a black screen.
>>> That is because when guest was during suspending, it called into Qemu
>>> and Qemu destroyed all resources and reset renderer. This made the
>>> display gone after guest resumed.
>>> So, I add a mechanism that when guest is suspending, it will notify
>>> Qemu, and then Qemu will not destroy resources. That can help guest's
>>> display come back.
>>> As discussed and suggested by Robert Beckett and Gerd Hoffmann on v1
>>> qemu's mailing list. Due to that mechanism needs cooperation between
>>> guest and host. What's more, as virtio drivers by design paravirt
>>> drivers, it is reasonable for guest to accept some cooperation with
>>> host to manage suspend/resume. So I request to add a new feature flag,
>>> so that guest and host can negotiate whenever freezing is supported or not.
>>> Jiqian Chen (1):
>>>   virtio-gpu: Add new feature flag VIRTIO_GPU_F_FREEZE_S3
>>>  device-types/gpu/description.tex | 42
>>> ++++++++++++++++++++++++++++++++
>>>  1 file changed, 42 insertions(+)
>> --
>> Best regards,
>> Jiqian Chen.

Best regards,
Jiqian Chen.

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