[virtio-dev] Re: [PATCH v2 0/2] transport-pci: Introduce legacy registers access using AQ

Parav Pandit
2023-05-10T16:13:51Z

On 5/10/2023 3:43 AM, Michael S. Tsirkin wrote:
> On Wed, May 10, 2023 at 03:01:25PM +0800, Jason Wang wrote:
>> On Wed, May 10, 2023 at 2:05 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>>>
>>> On Mon, May 08, 2023 at 10:23:39AM +0800, Jason Wang wrote:
>>>>> I thought so too originally. Unfortunately I now think that no, legacy is not
>>>>> going to be a byproduct of transport virtqueue for modern -
>>>>> it is different enough that it needs dedicated commands.
>>>>
>>>> If you mean the transport virtqueue, I think some dedicated commands
>>>> for legacy are needed. Then it would be a transport that supports
>>>> transitional devices. It would be much better than having commands for
>>>> a partial transport like this patch did.
>>>
>>> OK I am beginning to get what you are saying. So your criticism is
>>> this: what if device supports vq transport for modern, and we want to
>>> build a transitional device on top. how will that look. yes?
>>
>> Yes. I think it needs to be done through the transport virtqueue
>> otherwise the transport is not self-contained.
>
> I mean, any feature can be done over transport vq.
>
> But there is value in adding legacy commands to an otherwise
> modern device without reworking it completely to
> switch to a different transport.
>
Yes.

>
>>> A reasonable thing to include at least in the commit log. Parav?
>>>
>>> You are also asking what if the device uses transport vq,
>>> and we want transitional on top of that.
>>> It's a fair question but I don't exactly get why would
>>> this legacy support feature be wanted for the vq transport
>>> and not for other transports.
>>
>> Not sure I get the question, but all the existing transport support
>> legacy, if we want to have another, should the legacy support be a
>> must or not?
>
> This specific proposal is for tunneling legacy over admin vq.
> It can live alongside a normal modern VF, with hypervisor
> combining these to create a transitional device.
>
True.