On Fri, Aug 16, 2024 at 03:39:41PM +1000, David Gibson wrote:
> Based on Stefano's recent patch for faster tests.
> 
> Allow the user to specify which addresses are translated when used by
> the guest, rather than always being the gateway address or nothing.
> We also allow this remapping to go to the host's global address (more
> precisely the address assigned to the guest) rather than just host
> loopback.
> 
> Suggestions for better names for the new options in patches 20 & 22
> are most welcome.
> 
> Along the way to implementing that make many changes to clarify what
> various addresses we track mean, fixing a number of small bugs as
> well.
> 
> NOTE: there is a bug in 21/22 which breaks some of the passt_tcp perf
> tests.  I haven't managed to figure out why it's causing the problem,
> or even what the exact triggering conditions are (running the single
> stalling iperf alone doesn't do it).  Have to wrap up for today, so I
> thought I'd get this out for review anyway.

I've identified the bug here.  IMO, it's a pre-existing problem that
only works by accident at the moment.  The immediate fix is pretty
obvious, but it raises some broader questions

The problem arises because of the MTU changes we make in order to test
throughput with different packet sizes.  Specifically we change the
MTU to values < 1280, which implicitly disables IPv6 since it requires
an MTU >= 1280.  When we change the MTU back to a larger value IPv6 is
re-enabled, but some configuration has been lost in the meantime.

After the MTU is restored the guest reconfigures with NDP, but does
not re-DHCPv6.  That means the guest gets a SLAAC address in the right
prefix but not the exact /128 address we've tried to assign to it.
However, at least with the sequence of things we have in the tests,
the guest never sends any packets with the new address, so passt
doesn't update addr_seen.  When the inbound connection comes we send
it to the assigned address instead of the guest's actual address and
the guest rejects it.

This "worked" previously, because before this patch, passt would
translate the inbound connection to have source/dest as link-local
addresses.  We *do* have a current addr_ll_seen because (a) it won't
change if the guest doesn't change MAC and (b) when IPv6 is re-enabled
the NDP traffic the guest generates will have link-local addresses
that update addr_ll_seen.  With this patch, and a global address for
--map-host-loopback, we now need to send to addr_seen instead of
addr_ll_seen, hence exposing the bug.

In the short term, the obvious fix would be to re-run dhclient -6 in
the guest after we twiddle MTU but before running IPv6 tests.

This kind of opens a question about how hard we should try to
accomodate guests which don't configure themselves how we told them.
Personally I'd be ok with saying that nothing works if the guest
doesn't configure itself properly, thereby removing addr_seen and
addr_ll_seen entirely.  But I think, Stefano, you've been against that
idea in the past.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson