poor beacon/scan reliability with mac80211_hwsim

[James Prestwood <prestwoj@gmail.com>]

Hi,

We use mac80211_hwsim and our own 'hwsim' daemon to test conditions
like poor signal strength or dropped frames. For quite a while we've
noticed very poor reliability related to scanning when
HWSIM_CMD_REGISTER is used to process frames. Scan results are just
empty.

We've put in some work arounds like only registering for tests that
absolutely need it and repeatedly scanning until the expected network
is found, but there are cases where this is not possible.

I'm hoping for some ideas on how to actually fix this problem rather
than continue trying to come up with workarounds. I have tried removing
any frame processing (just an empty function) and noticed the problem
still occurs, basically just calling HWSIM_CMD_REGISTER causes these
problems.

I will admit this seems to happen more on slower systems, like inside a
virtual machine environment, or in tests which create more than just a
few radios (like ~5-6+) so it does seem like mac80211_hwsim/wmediumd
processing the frame is just taking too long for beacons.

Thanks,
James