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

[Cornelia Huck <cohuck@redhat.com>]

On Tue, Aug 03 2021, Jason Wang <jasowang@redhat.com> wrote:

> å 2021/8/3 äå2:28, Cornelia Huck åé:
>> 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.
>
>  ÂÂÂÂÂÂÂ le32 device_feature_select;ÂÂÂÂ /* read-write */
>  ÂÂÂÂÂÂÂ le32 device_feature;ÂÂÂÂÂÂÂÂÂÂÂ /* read-only for driver */
>  ÂÂÂÂÂÂÂ le32 driver_feature_select;ÂÂÂÂ /* read-write */
>  ÂÂÂÂÂÂÂ le32 driver_feature;ÂÂÂÂÂÂÂÂÂÂÂ /* 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.