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

[Jason Wang <jasowang@redhat.com>]

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.

Thanks