iflib: netmap: add per-tx-queue netmap support

[vmaffione]

Summary

Add support for those cases where only a subset of the TX rings are opened, and where RX rings are not opened.
Rather than simply checking IFCAP_NETMAP, also use the netmap_kring_on() helper or the return value of netmap_tx_irq().
In the interrupt handler, in particular, there is no need to call isc_credits_update.

Test Plan

Tested with pkt-gen, bridge, vale-ctl on em devices within a VM.

[vmaffione]

I suspect something went wrong with the email notification, so this is just a ping message to make sure reviewers are aware of this patch.

[gallatin]

Can you add context please ? I upload patches using git diff -U9999999 to get the full file to show up.

[vmaffione]

Uploaded diff context

[gallatin]

sys/net/iflib.c

3763

I have a concern is for performance in the netmap compiled in, but inactive case.

In this case, it seems that you've replaced a very cheap check of an ifp flag IFCAP_NETMAP (which is almost certain to be in cache) with a function call which then de-references ifp->if_netmap, which is likely to be cold if netmap is disabled.

Why did you need to remove the IFCAP_NETMAP check?

[vmaffione]

Be more cache-friendly by checking IFCAP_NETMAP before calling netmap_tx_irq.

[vmaffione]

sys/net/iflib.c

3763

That's a very good point, thanks.
Yes, we can always check IFCAP_NETMAP before actually checking if the specific netmap ring is on (with netmap_tx_irq or netmap_kring_on).
I removed that check just to simplify the code, but I do agree that this causes unnecessary cache misses (and a function call) in the (normal) case where netmap is not in use.
On the contrary, there practically no impact on the case where netmap is in use.

[gallatin]

Thank you for making this change!