On Tue, Apr 25, 2023 at 01:02:38PM +0000, Alvaro Karsz wrote: > > > 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 > > > > why MRG_RXBUF? what does it matter? > > > > You're right, it should be blocked only when ring < 2. > Or we should let this pass, and let the device figure out that MRG_RXBUF is meaningless with 1 entry.. yep, later I think. > > > 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. > > > > Using which APIs? What does peek_vqs_len do and why does it matter that > > it is super early? > > > > We peek at the lengths using a new virtio_config.h function that calls a transport specific callback. > We renegotiate calling the new, exported virtio core function. > > peek_vqs_len fills an array of u16 variables with the max length of every VQ. > > The idea here is not to fail probe. > So we start probe, check if the ring is small, renegotiate the features and then continue with the new features. > This needs to be super early because otherwise, some virtio_has_feature calls before re-negotiating may be invalid, meaning a lot of reconfigurations. > > > > * 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. > > > 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? > > > > Lots of work spilling over to transports. > > > > And I especially don't like that it slows down boot on good path. > > Yes, but I don't think that this is really significant. > It's just a call to the transport to get the length of the VQs. With lots of VQs that is lots of exits. > If ring is not small, we continue as normal. > If ring is small, we renegotiate and continue, without failing probe. > > > > > I have the following idea: > > - add a blocked features value in virtio_device > > - before calling probe, core saves blocked features > > - if probe fails, checks blocked features. > > if any were added, reset, negotiate all features > > except blocked ones and do the validate/probe dance again > > > > > > This will mean mostly no changes to drivers: just check condition, > > block feature and fail probe. > > > > I like the idea, will try to implement it. > > Thanks, _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
