Re: [virtio-comment] [PATCH v2] Introduction of Virtio Network device notifications coalescing feature

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

I forgot to address this point in your previous  email:

> My guess is that VIRTIO_F_EVENT_IDX and
> *-max-frames have similar effects in suppressing notifications.

One big difference is that if you want to ask the device to send an
interrupt every 20 packets, you should set VIRTIO_F_EVENT_IDX to 20.
Then when an interrupt is received, change it to 40, then to 60..

Using the notifications coalescing feature, you just need to set the
max-frames values once, and that's it.

> I'm thinking about the scenario where EVENT_IDX and max-frames are not
> multiples and the coalesce timer is disabled (0). Can the driver be in a
> situation where is waits forever?

The timer can't be disabled.
Without the timer, you'll have problems even without  EVENT_IDX/
notification suppressions.
Assume a scenario when max-frames is 10, and the device receives only
5 packets, and then the network is silent.
In this case, these packets will never reach the driver.

So, the timer must always work if a device supports this feature.

timer=0 should mean that an interrupt is sent if 0 usecs elapsed since
the last interrupt, which always will be "yes".


> I think the max-frames state should only be reset when the Used Buffer
> Notification is sent. That way a notification is sent after 3 tx buffers
> have been used and the driver will be able to make progress.

It's possible to ask the device to reset the coalescing counters only
when an interrupt is sent, but I think that it is not required because
of the coalescing timer.


Alvaro