Re: [virtio-dev] Re: [PATCH v3] virtio-net: support the virtqueue coalescing moderation

[Alvaro Karsz <alvaro.karsz@solid-run.com>]

> > Yes, max_packets and max_usecs SHOULD be reset to 0.
> > When the virtqueue is re-enabled, it means that notification conditions are met after each packet is sent/received.
> >
> > As described by Alvaro in "[PATCH v5] virtio-net: Fix and update VIRTIO_NET_F_NOTF_COAL feature":
> > "+When the device has \field{max_usecs} = 0 or \field{max_packets} = 0, the notification conditions are met after every packet received/sent."
>
> Oh. Hmm.
>
> What Alvaro's patch does not describe is what happens when VQ is reset.
>
> Alvaro you said you have hardware implementing this right?
> How does the command interact with vq reset?
>

The device doesn't offer VIRTIO_F_RING_RESET at the moment.

>
> > > What about VIRTIO_NET_CTRL_NOTF_COAL?
> >
> > I think it should be handled in the same way as the above VQ_SET, that is, reset the corresponding virtqeuue parameters to 0.
>
> sounds consistent. but the problem is, I don't think this is
> how we previously behaved. and RING_RESET is out in 1.2.
> So we need something compatible. I am sorry.
>
> I suspect that instead we can say that existing hardware has a default set of
> parameters for tx and rx. And global commands change that
> besides changing all enabled VQs.
>
> That is a side effect beyond just changing all VQs.
>
> Hmm.
> Alvaro?
>

This is indeed a good point.
We mention the device reset case, but nothing about VQ reset.

I feel that no matter how we handle this, we break something.

Having default coalescing values may collide with "Upon reset, a
device MUST initialize all coalescing parameters to 0."

We can say that VQ reset doesn't affect the "global parameters" and a
device reset does, but this collides with "Device Requirements:
Virtqueue Reset".

In fact, resetting coalescing values after vq reset may be derived
from "Upon reset, a device MUST initialize all coalescing parameters
to 0".
This is consistent with "Device Requirements: Virtqueue Reset".

I can add in my patch some clarifications.

This will break the linux virtio_net ethtool implementation a little,
we'll need to fix it.