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

[Jason Wang <jasowang@redhat.com>]

在 2023/5/11 20:54, Michael S. Tsirkin 写道:
> On Thu, May 11, 2023 at 03:04:40PM +0800, Jason Wang wrote:
>> On Wed, May 10, 2023 at 3:43 PM Michael S. Tsirkin <mst@redhat.com> 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.
>> There's probably no need for a rework since legacy is not complicated.
>> More below.
>>
>>>
>>>>> 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.
>> Exactly, but what I meant here is
>>
>> If we decide to use the admin vq, is there any good reason to tie it
>> to PCI if we don't want to tunneling PCI over adminq?
>>
>> Why not simply invent individual commands to access legacy facilities
>> like commands to access like what transport virtqueue did for modern
>> device?:
>>
>> 1) device features
>> 2) driver features
>> 3) queue address
>> 4) queue size
>> 5) queue select
>> 6) queue notify
>> 7) device status
>> 8) ISR status
>> 9) config msix
>> 10) queue msix
>> 11) device configuration space
>>
>> It focuses on the facilities instead of transport specific details
>> like registers (we don't even need legacy registers in this case), I
>> gives more deterministic behavior so we don't need to care about the
>> cross registers read/write.
> This needs thought, it is definitely more work.  Effort that could be
> maybe spent on new features.  What is the motivation
> here? supporting legacy mmio guests?


Probably. It tries to make legacy transport independent, and it's the 
way that how transport virtqueue want to be.

Thanks


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