On Tue, Apr 25, 2023 at 11:11:54AM +0000, Alvaro Karsz wrote: > > > So, let's add some funky flags in virtio device to block out > > > features, have core compare these before and after, > > > detect change, reset and retry? > > > > In the virtnet case, we'll decide which features to block based on the ring size. > > 2 < ring < MAX_FRAGS + 2 -> BLOCK GRO + MRG_RXBUF > > ring < 2 -> BLOCK GRO + MRG_RXBUF + CTRL_VQ > > > > So we'll need a new virtio callback instead of flags. > > > > Furthermore, other virtio drivers may decide which features to block based on parameters different than ring size (I don't have a good example at the moment). > > So maybe we should leave it to the driver to handle (during probe), > and offer a virtio core function to re-negotiate the features? > > > > In the solution I'm working on, I expose a new virtio core function that resets the device and renegotiates the received features. > > + A new virtio_config_ops callback peek_vqs_len to peek at the VQ lengths before calling find_vqs. (The callback must be called after the features negotiation) > > > > So, the flow is something like: > > > > * Super early in virtnet probe, we peek at the VQ lengths and decide if we are > > using small vrings, if so, we reset and renegotiate the features. > > * We continue normally and create the VQs. > > * We check if the created rings are small. > > If they are and some blocked features were negotiated anyway (may occur if > > the re-negotiation fails, or if the transport has no implementation for > > peek_vqs_len), we fail probe. > > Small fix: if the re-negotiation fails, we fail probe immediately. > The only way to negotiate blocked features with a small vring is if the transport has no implementation for peek_vqs_len. with my idea, you can go iteratively: fail one condition, core will retry with a feature blocked, we can block more, retry again. up to 64 times :) > > If the ring is small and the features are ok, we mark the virtnet device as > > vring_small and fixup some variables. > > > > > > peek_vqs_len is needed because we must know the VQ length before calling init_vqs. > > > > During virtnet_find_vqs we check the following: > > vi->has_cvq > > vi->big_packets > > vi->mergeable_rx_bufs > > > > But these will change if the ring is small.. > > > > (Of course, another solution will be to re-negotiate features after init_vqs, but this will make a big mess, tons of things to clean and reconfigure) > > > > > > The 2 < ring < MAX_FRAGS + 2 part is ready, I have tested a few cases and it is working. > > > > I'm considering splitting the effort into 2 series. > > A 2 < ring < MAX_FRAGS + 2 series, and a follow up series with the ring < 2 case. > > > > I'm also thinking about sending the first series as an RFC soon, so it will be more broadly tested. > > > > What do you think? > > _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
