Re: [virtio-comment] Re: [PATCH v3 6/8] admin: Add theory of operation for write recording commands

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

On 11/15/2023 7:52 PM, Michael S. Tsirkin wrote:
> On Wed, Nov 15, 2023 at 04:42:56PM +0800, Zhu, Lingshan wrote:
>>
>> On 11/15/2023 4:05 PM, Michael S. Tsirkin wrote:
>>> On Wed, Nov 15, 2023 at 03:59:16PM +0800, Zhu, Lingshan wrote:
>>>> On 11/15/2023 3:51 PM, Michael S. Tsirkin wrote:
>>>>> On Wed, Nov 15, 2023 at 12:05:59PM +0800, Zhu, Lingshan wrote:
>>>>>> On 11/14/2023 4:27 PM, Michael S. Tsirkin wrote:
>>>>>>> On Tue, Nov 14, 2023 at 03:34:32PM +0800, Zhu, Lingshan wrote:
>>>>>>>>>> So I can't
>>>>>>>>>> believe it has good performance overall. Logging via IOMMU or using
>>>>>>>>>> shadow virtqueue doesn't need any extra PCI transactions at least.
>>>>>>>>> On the other hand they have an extra CPU cost.  Personally if this is
>>>>>>>>> coming from a hardware vendor, I am inclined to trust them wrt PCI
>>>>>>>>> transactions.  But anyway, discussing this at a high level theoretically
>>>>>>>>> is pointless - whoever bothers with actual prototyping for performance
>>>>>>>>> testing wins, if no one does I'd expect a back of a napkin estimate
>>>>>>>>> to be included.
>>>>>>>> if so, Intel has released productions implementing these interfaces years
>>>>>>>> ago,
>>>>>>>> see live migration in 4.1. IFCVF vDPA Implementation,
>>>>>>>> https://doc.dpdk.org/guides-21.11/vdpadevs/ifc.html
>>>>>>>> and
>>>>>>> That one is based on shadow queue, right? Which I think this shows
>>>>>>> is worth supporting.
>>>>>> Yes, it is shadow virtqueue, I assume this is already mostly done,
>>>>>> do you see any gaps we need to address in our series that we should work on?
>>>>>>
>>>>>> Thanks
>>>>> There were a ton of comments posted on your series.
>>>> Hope I didn't miss anything. I see your latest comments are about vq states,
>>>> as replied before, I think we can record the states by two le16 and the
>>>> in-flight
>>>> descriptor tracking facility.
>>> I don't know why you need the le16. in-flight tracking should be enough.
>>> And given it needs DMA I would try really hard to actually use
>>> admin commands for this.
>> we need to record the on-device avail_idx and used_idx, or
>> how can the destination side know the device internal values.
> Again you never documented what state the device is in so I can't really
> say for sure.  But generally whenever a buffer is used the internal
> values are written out to memory.
This is the state of a virtqueue, in my series I have defined what is
vq state in [PATCH V2 1/6] virtio: introduce virtqueue state,
and give an example of PCI implementation.

In short, for split vq it is last_avail_idx and in-flight descriptors.

I humbly request an explicit list of missing gaps, so that I can improve 
my V3

Thanks
>
>>>> For this shadow virtqueue, do you think I should address this in my V4?
>>>> Like saying: acknowledged control commands through the control virtqueue
>>>> should be recorded, and we want to use shadow virtqueue to track dirty
>>>> pages.
>>> What you need to do is actually describe what do you expect the device
>>> to do when it enters this suspend state. since you mention control
>>> virtqueue then it seems that there needs to be a device type
>>> specific text explaining the behaviour. If so this implies there
>>> needs to be a list of device types that support suspend
>>> until someone looks at each type and documents what it does.
>> On a second thought, shadow vqs are hypervisor behaviors, maybe should not
>> be
>> described in this device spec.
>>
>> Since SUSPEND is in device status, so for now I see every type of device
>> implements
>> device_status should support SUSPEND. This should be a general facility.
>


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