Re: [PATCH net] virtio-net: reject small vring sizes

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> 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.
   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:

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?
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