[virtio-comment] Re: [RFC PATCH v2 1/2] Add virtio Admin Virtqueue specification

[Max Gurtovoy <mgurtovoy@nvidia.com>]

On 8/3/2021 9:51 AM, Cornelia Huck wrote:
> On Tue, Aug 03 2021, Jason Wang <jasowang@redhat.com> wrote:
>
>> 在 2021/8/3 下午2:28, Cornelia Huck 写道:
>>> On Mon, Aug 02 2021, "Michael S. Tsirkin" <mst@redhat.com> wrote:
>>>
>>>> On Mon, Aug 02, 2021 at 07:03:11PM +0300, Max Gurtovoy wrote:
>>>>> There is no much bits left in the generic feature field for all the features
>>>>> we would like to add.
>>>>> I mentioned only 5-6 in the above example and it will bring us to bit 46
>>>>> already.
>>>>>
>>>>> please think of 5-10 years from today.
>>>>>
>>>> IIUC nothing prevents adding more once we exhaust 64 bits. IMHO it's actually
>>>> pretty important to make sure the feature negotiation works well
>>>> and covers relevant usecases. If we have limitations preventing that
>>>> I'd like to at least try to fix that not replacing feature negotiation with
>>>> something else.
>>> I recall that we had a discussion about that years ago when we
>>> introduced VERSION_1; we explicitly agreed that we can extend features
>>> beyond 64 bit once we need it. (A quick search did not turn up that mail
>>> exchange, though.)
>>>
>> E.g PCI transport has feature_select.
>>
>>           le32 device_feature_select;     /* read-write */
>>           le32 device_feature;            /* read-only for driver */
>>           le32 driver_feature_select;     /* read-write */
>>           le32 driver_feature;            /* read-write */
>>
>> Technically it can support 32*32 different features.
> MMIO uses a similar scheme; CCW uses
>
> struct virtio_feature_desc {
>         le32 features;
>         u8 index;
> };
>
> so there's plenty of room to spare.

Feature negotiation can be done in admin q level. You don't want this 
branches in the spec per each transport or device type.

We have an admin command to get all device admin q features and we can 
create a command to set all the driver supported.

Again, I don't see the point of doing that. If a device is supporting 
command_A, the driver should be able to use it. Telling the device "hi, 
I'm going to use command_A in the future" is redundant.

The device knows already how to handle it.


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