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

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

> Not good. I feel we must come up with spec that is backwards compatible.
> Hmm, maybe this is why Parav kept talking about modes.
> I did not realize at the time, sorry Parav.
>
> I still feel modes are not the best way to describe things so I propose this:
> - in addition to per vq parameters, device that supports global TX/RX
>   commands and ring reset maintains two sets of default parameters: for RX and TX
> - existing commands change default and change all enabled vqs
>   of the correct type (RX/TX) to the same value
> - vq reset changes a vq to the default
> - device reset changes defaults to 0 and changes all vqs to  0
>
> note how defaults are only used for ring reset.  is "vq reset parameter"
> a better name? I feel we will repeat "reset" too many times in a
> sentence if we call it that though.
>
> So fundamentally the only change is with RING_RESET, then
> default is not always 0, it can be set by the global command.

I like the idea, maintaining default coalescing parameters is
compatible with vq reset.

"After a queue has been reset by the driver, the device MUST NOT
execute any requests from that virtqueue, or notify the driver for it.
The device MUST reset any state of a virtqueue to the *default state*,
including the available state and the used state."

I wonder if some of that should be included in my patch.
Most of that is not relevant before introducing the per vq command.
So maybe I could just mention in the device conformance that:
"Coalescing parameters MUST/SHOULD persist a virtqueue reset."

And then Heng can edit/add on top of that.


Now that the existing commands do more than just iterating through all
the virtqueues, maybe we should be able to modify all virtqueues with
VIRTIO_NET_F_VQ_NOTF_COAL.

I think that using a special vqn is messy, we can split "le32
reserved" to "le16 flags" and "le16 reserved".
Then we can have:
- A flag to apply parameters to all receive VQs, vqn is ignored in
this case. (same as VIRTIO_NET_CTRL_NOTF_COAL_RX_SET but without
changing the default parameters).
- A flag to apply parameters to all transmit VQs, vqn is ignored in
this case. (same as VIRTIO_NET_CTRL_NOTF_COAL_TX_SET but without
changing the default parameters).

And maybe a flag to apply to all vqs (transmit + receive), I actually
am not sure how useful this is..