Message-Id: <20180111235905.10110-1-dja@axtens.net>
Date:   Fri, 12 Jan 2018 10:59:05 +1100
From:   Daniel Axtens <dja@...ens.net>
To:     netdev@...r.kernel.org
Cc:     Daniel Axtens <dja@...ens.net>,
        Thomas Falcon <tlfalcon@...ux.vnet.ibm.com>,
        Yuval Mintz <Yuval.Mintz@...ium.com>
Subject: [PATCH v2] bnx2x: disable GSO where gso_size is too big for hardware

If a bnx2x card is passed a GSO packet with a gso_size larger than
~9700 bytes, it will cause a firmware error that will bring the card
down:

bnx2x: [bnx2x_attn_int_deasserted3:4323(enP24p1s0f0)]MC assert!
bnx2x: [bnx2x_mc_assert:720(enP24p1s0f0)]XSTORM_ASSERT_LIST_INDEX 0x2
bnx2x: [bnx2x_mc_assert:736(enP24p1s0f0)]XSTORM_ASSERT_INDEX 0x0 = 0x00000000 0x25e43e47 0x00463e01 0x00010052
bnx2x: [bnx2x_mc_assert:750(enP24p1s0f0)]Chip Revision: everest3, FW Version: 7_13_1
... (dump of values continues) ...

Detect when gso_size + header length is greater than the maximum
packet size (9700 bytes) and disable GSO. For simplicity and speed
this is approximated by comparing gso_size against 9200 and assuming
no-one will have more than 500 bytes of headers.

This raises the obvious question - how do we end up with a packet with
a gso_size that's greater than 9700? This has been observed on an
powerpc system when Open vSwitch is forwarding a packet from an
ibmveth device.

ibmveth is a bit special. It's the driver for communication between
virtual machines (aka 'partitions'/LPARs) running under IBM's
proprietary hypervisor on ppc machines. It allows sending very large
packets (up to 64kB) between LPARs. This involves some quite
'interesting' things: for example, when talking TCP, the MSS is stored
the checksum field (see ibmveth_rx_mss_helper() in ibmveth.c).

Normally on a box like this, there would be a Virtual I/O Server
(VIOS) partition that owns the physical network card. VIOS lets the
AIX partitions know when they're talking to a real network and that
they should drop their MSS. This works fine if VIOS owns the physical
network card.

However, in this case, a Linux partition owns the card (this is known
as a NovaLink setup). The negotiation between VIOS and AIX uses a
non-standard TCP option, so Linux has never supported that.  Instead,
Linux just supports receiving large packets. It doesn't support any
form of messaging/MSS negotiation back to other LPARs.

To get some clarity about where the large MSS was coming from, I asked
Thomas Falcon, the maintainer of ibmveth, for some background:

"In most cases, large segments are an aggregation of smaller packets
by the Virtual I/O Server (VIOS) partition and then are forwarded to
the Linux LPAR / ibmveth driver. These segments can be as large as
64KB. In this case, since the customer is using Novalink, I believe
what is happening is pretty straightforward: the large segments are
created by the AIX partition and then forwarded to the Linux
partition, ... The ibmveth driver doesn't do any aggregation itself
but just ensures the proper bits are set before sending the frame up
to avoid giving the upper layers indigestion."

It is possible to stop AIX from sending these large segments, but it
requires configuration on each LPAR. While ibmveth's behaviour is
admittedly weird, we should fix this here: it shouldn't be possible
for it to cause a firmware panic on another card.

Cc: Thomas Falcon <tlfalcon@...ux.vnet.ibm.com> # ibmveth
Cc: Yuval Mintz <Yuval.Mintz@...ium.com> # bnx2x
Thanks-to: Jay Vosburgh <jay.vosburgh@...onical.com> # veth info
Signed-off-by: Daniel Axtens <dja@...ens.net>

---
v2: change to a feature check as suggested by Eric Dumazet.

---
 drivers/net/ethernet/broadcom/bnx2x/bnx2x_main.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/net/ethernet/broadcom/bnx2x/bnx2x_main.c b/drivers/net/ethernet/broadcom/bnx2x/bnx2x_main.c
index 7b08323e3f3d..bab909b5d7a2 100644
--- a/drivers/net/ethernet/broadcom/bnx2x/bnx2x_main.c
+++ b/drivers/net/ethernet/broadcom/bnx2x/bnx2x_main.c
@@ -12934,6 +12934,17 @@ static netdev_features_t bnx2x_features_check(struct sk_buff *skb,
 					      struct net_device *dev,
 					      netdev_features_t features)
 {
+	/*
+	 * A skb with gso_size + header length > 9700 will cause a
+	 * firmware panic. Drop GSO support.
+	 *
+	 * To avoid costly calculations on all packets (and because
+	 * super-jumbo frames are rare), allow 500 bytes of headers
+	 * and just disable GSO if gso_size is greater than 9200.
+	 */
+	if (unlikely(skb_is_gso(skb) && skb_shinfo(skb)->gso_size > 9200))
+		features &= ~NETIF_F_GSO_MASK;
+
 	features = vlan_features_check(skb, features);
 	return vxlan_features_check(skb, features);
 }
-- 
2.14.1

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