Re: [PATCH net-next] net: ethernet: ti: cpsw: don't flush mcast entries while switch promisc mode

On 10/19/18 7:04 AM, Ivan Khoronzhuk wrote:

On Thu, Oct 18, 2018 at 07:03:06PM -0500, Grygorii Strashko wrote:

quoted

On 10/18/18 1:00 PM, Ivan Khoronzhuk wrote:

quoted

No need now to flush mcast entries in switch mode while toggling to
promiscuous mode. It's not needed as vlan reg_mcast = ALL_PORTS
and mcast/vlan ports = ALL_PORTS, the same happening for vlan
unreg_mcast, it's set to ALL_PORT_MASK just after calling promisc
mode routine by calling set allmulti. I suppose main reason to flush
them is to use unreg_mcast to receive all to host port. Thus, now, all
mcast packets are received anyway and no reason to flush mcast entries
unsafely, as they were synced with __dev_mc_sync() previously and are
not restored. Another way is to _dev_mc_unsync() them, but no need.

User have possibility to add additional mcast entries or edit existing in switch-mode, which is now done using custom tool. So, Host in promisc
mode will not receive packets for mcast address X if port mask for this
addr set to (ALL_PORTS - HOST_PORT). Am I missing smth?

I didn't take into account the custom tool changing entries directly,
but even in this case there is at least a couple of interesting
questions:

1) Before the patch applied only several days ago -
   5da1948969bc1991920987ce4361ea56046e5a98
   "ti: cpsw: fix lost of mcast packets while rx_mode update"
   It was impossible to do correctly anyway, as all mcast entries not
   in the mc list were flushed (after rx_mode cb), by:
   cpsw_ale_flush_multicast(cpsw->ale, ALE_ALL_PORTS, vid);
   and those in mc, rewritten by adding them back in corrected form.
   ... or this cb was not supposed to be called at all ...

It's not allowed to manipulate ALE table in dual_mac mode, so your
patches are safe in dual_mac mode. For switch-mode (unless we  move
forward with switch dev) standard linux interfaces allows create
default mcast entries which then (if required) corrected using custom
tool now.

2) What is the reason to add mcast switch entires
   (ALL_PORTS - HOST_PORT) if its function is added anyway by
   unreq_mcast & (ALL_PORTS - HOST_PORT) ?
   So, doesn't matter it's added or not - it will work :-|.

because in switch mode not all traffic directed to the Host port -
only in promisc mode. Reason safety and performance - Host should not
receive traffic which is not designated for it.

promiscuous in switch mode:
- disables learning
- enables unicast flooding to Host port
- enables unregistered multi-cast flooding to the Host port
In other words, CPSW will continue forwarding packets between P1&P2, but also will "duplicate" packets to Host port. This will work only for
vlans which have host port as member.

3) Even so, toggling promisc mode will clear all these changes anyway,
   even I will call _dev_mc_unsync() after flushing them.

there can be records which are not under control of Linux now.

4) If user can tune ALE table by hand, what stops him do it after moving
   to promisc mode, seems he knows what he's doing?

5) It could be possible only for not default vlan entries, but mcast
   vlan support is not supported yet. Who is gona restore those
   entries after promisc off?

This behaviour is arguable, and flushing mcast entries can bring more
issues then leaving. For me it doesn't matter, I can archive the same
by adding after flush one line, it's even shorter:
__dev_mc_unsync(priv->ndev, NULL);

Again, unless we move forward with switch dev you can't assume that
Linux stack has full control over ALE table. Sry, hence this patch is
not a fix and can introduce changes in current behavior and cause
regression reports - NACK.

-- 
regards,
-grygorii