Re: [PATCH net-next v4 11/11] net: dsa: realtek: rtl8365mb: multiple cpu ports, non cpu extint

[Vladimir Oltean <olteanv@gmail.com>]

On Mon, Jan 24, 2022 at 01:42:42PM -0800, Jakub Kicinski wrote:
> On Mon, 24 Jan 2022 22:56:07 +0200 Vladimir Oltean wrote:
> > On Mon, Jan 24, 2022 at 11:38:12AM -0800, Jakub Kicinski wrote:
> > > On Mon, 24 Jan 2022 21:08:45 +0200 Vladimir Oltean wrote:
> > > > So before we declare that any given Ethernet driver is buggy for declaring
> > > > NETIF_F_IP_CSUM | NETIF_F_IPV6_CSUM and not checking that skb->csum_start
> > > > points where it expects it to (taking into consideration potential VLAN
> > > > headers, IPv6 extension headers),
> > >
> > > Extension headers are explicitly not supported by NETIF_F_IPV6_CSUM.
> > >
> > > IIRC Tom's hope was to delete NETIF_F_IP*_CSUM completely once all
> > > drivers are converted to parsing and therefore can use NETIF_F_HW_CSUM.
> >
> > IIUC, NETIF_F_IP*_CSUM vs NETIF_F_HW_CSUM doesn't make that big of a
> > difference in terms of what the driver should check for, if the hardware
> > checksum offload engine can't directly be given the csum_start and
> > csum_offset, wherever they may be.
> >
> > > > is there any driver that _does_ perform these checks correctly, that
> > > > could be used as an example?
> > >
> > > I don't think so. Let me put it this way - my understanding is that up
> > > until now we had been using the vlan_features, mpls_features etc to
> > > perform L2/L2.5/below-IP feature stripping. This scales poorly to DSA
> > > tags, as discussed in this thread.
> > >
> > > I'm suggesting we extend the kind of checking we already do to work
> > > around inevitable deficiencies of device parsers for tunnels to DSA
> > > tags.
> >
> > Sorry, I'm very tired and I probably don't understand what you're
> > saying, so excuse the extra clarification questions.
> >
> > The typical protocol checking that drivers with NETIF_F_HW_CSUM do seems
> > to be based on vlan_get_protocol()/skb->protocol/skb_network_header()/
> > skb_transport_header() values, all of which make DSA invisible. So they
> > don't work if the underlying hardware really doesn't like seeing an
> > unexpected DSA header.
> >
> > When you say "I'm suggesting we extend the kind of checking we already do",
> > do you mean we should modify the likes of e1000e and igb such that, if
> > they're ever used as DSA masters, they do a full header parse of the
> > packet (struct ethhdr :: h_proto, check if VLAN, struct iphdr/ipv6hdr,
> > etc.) instead of the current logic?
>
> That was my thinking, yes. The exact amount of work depends on the
> driver, I believe that more recent Intel parts (igb, ixgbe and newer)
> pass a L3 offset to the HW. They treat L2 as opaque, ergo no patches
> needed. At a glance e1000e passes the full skb_checksum_start_offset()
> to HW, so likely even better.

Ah, right, I missed that. I agree that a driver that uses
skb->csum_start is very likely to work unmodified with DSA headers
(not trailers). I didn't notice this in e1000 because I was just
searching for csum_start.

> It's only drivers for devices which actually want to parse the Ethertype
> that would need extra checks. (Coincidentally such devices can't support
> MPLS given the lack of L3 indication in the frame.)
>
> > It will be pretty convoluted unless
> > we have some helper. Because if I follow through, for a DSA-tagged IP
> > packet on xmit, skb->protocol is certainly htons(ETH_P_IP):
> >
> > ntohs(skb->protocol) = 0x800, csum_offset = 16, csum_start = 280, skb_checksum_start_offset = 54, skb->network_header = 260, skb_network_header_len = 20
> >
> > skb_dump output:
> > skb len=94 headroom=226 headlen=94 tailroom=384
> > mac=(226,34) net=(260,20) trans=280
> > shinfo(txflags=0 nr_frags=0 gso(size=0 type=1 segs=1))
> > csum(0x100118 ip_summed=3 complete_sw=0 valid=0 level=0)
> > hash(0x7710ee84 sw=0 l4=1) proto=0x0800 pkttype=0 iif=0
> > dev name=eno2 feat=0x00020100001149a9
> > sk family=2 type=1 proto=6
> > skb headroom: 00000000: 6c 00 03 02 64 65 76 00 fe ed ca fe 28 00 00 00
> > ...(junk)...
> > skb headroom: 000000e0: 5f 43
> >                         20 byte DSA tag
> >                         |
> >                         v
> > skb linear:   00000000: 88 80 00 0a 80 00 00 00 00 00 00 00 08 00 30 00
> >                                     skb_mac_header()
> >                                     |
> >                                     v
> > skb linear:   00000010: 00 00 00 00 68 05 ca 92 af 20 00 04 9f 05 f6 28
> >                               skb_network_header()
> >                               |
> >                               v
> > skb linear:   00000020: 08 00 45 00 00 3c 26 47 40 00 40 06 00 49 0a 00
> >                                           skb_checksum_start_offset
> >                                           |
> >                                           |                       csum_offset
> >                                           v                       v
> > skb linear:   00000030: 00 2c 0a 00 00 01 b6 08 14 51 11 1f 91 4f 00 00
> > skb linear:   00000040: 00 00 a0 02 fa f0 14 5b 00 00 02 04 05 b4 04 02
> > skb linear:   00000050: 08 0a 2e 00 e5 b8 00 00 00 00 01 03 03 07
>
> Oof, so in this case the DSA tag is _before_ the skb_mac_header()?
> Or the prepend is supposed to be parsable as a Ethernet header?
> Seems like any device that can do csum over this packet must already
> use L3/L4 offsets or have explicit knowledge of DSA, right?

I'm sorry, there's a mistake, skb_mac_header() points to the DSA tag
here (and skb_mac_header_len is 34), I wanted to say "real MAC header"
but failed to do so. This case shouldn't pose any special problems.

> > I don't know, I just don't expect that non-DSA users of those drivers
> > will be very happy about such changes. Do these existing protocol
> > checking schemes qualify as buggy?
>
> Unfortunate reality of the checksum offloads is that most drivers for
> devices which parse on Tx are buggy, it's more of a question of whether
> anyone tried to use an unsupported protocol stack :( Recent example
> that comes to mind is 1698d600b361 ("bnxt_en: Implement
> .ndo_features_check().").

Nice hook this ndo_features_check! In my reading of validate_xmit_skb()
I went right past it.

> > If this is the convention that we want to enforce, then I can't really
> > help Luiz with fixing the OpenWRT mtk_eth_soc.c - he'll have to figure
> > out a way to parse the packets for which his hardware will accept the
> > checksumming offload, and call skb_checksum_help() otherwise.
> >
> > > We can come up with various schemes of expressing capabilities
> > > between underlying driver and tag driver. I'm not aware of similar
> > > out-of-band schemes existing today so it'd be "DSA doing it's own
> > > thing", which does not seem great.
> >
> > It at least seems less complex to me, and less checking in the fast path
> > if I understand everything that's been said correctly.
>
> I understand, I'm primarily trying to share some context and prior work.
> I don't mean to nack all other approaches.
>
> I believe writing a parser matching the device behavior would be easier
> for a driver author than interpreting the runes of our csum offload API
> and getting thru the thicket of all the bits. If that's not the case my
> argument is likely defeated.

Ok, writing a parser might be needed if the DSA master is going, for
some reason, to support TX checksum offloading with some DSA headers but
not with others.

If that is not the case, and that Ethernet controller simply doesn't
support TX checksumming unless it's the plain old Ethernet {+ VLAN} +
IP/IPv6 + TCP/UDP, then this blanket patch below should fix the problem
almost elegantly, and parsing is a useless complication (warning, not
even compile-tested!):

-----------------------------[ cut here ]-----------------------------
From 5ef3d3cd8441d756933558212f518f48754c64d9 Mon Sep 17 00:00:00 2001
From: Vladimir Oltean <vladimir.oltean@nxp.com>
Date: Tue, 25 Jan 2022 00:16:57 +0200
Subject: [PATCH] ramips: ethernet: ralink: disable TX checksumming on DSA
 masters

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
---
 .../files/drivers/net/ethernet/ralink/mtk_eth_soc.c  | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/target/linux/ramips/files/drivers/net/ethernet/ralink/mtk_eth_soc.c b/target/linux/ramips/files/drivers/net/ethernet/ralink/mtk_eth_soc.c
index e07e5ed5a8f8..6ed9bc5942fd 100644
--- a/target/linux/ramips/files/drivers/net/ethernet/ralink/mtk_eth_soc.c
+++ b/target/linux/ramips/files/drivers/net/ethernet/ralink/mtk_eth_soc.c
@@ -31,6 +31,7 @@
 #include <linux/io.h>
 #include <linux/bug.h>
 #include <linux/netfilter.h>
+#include <net/dsa.h>
 #include <net/netfilter/nf_flow_table.h>
 #include <linux/of_gpio.h>
 #include <linux/gpio.h>
@@ -1497,6 +1498,16 @@ static int fe_change_mtu(struct net_device *dev, int new_mtu)
 	return fe_open(dev);
 }
 
+static netdev_features_t fe_features_check(struct sk_buff *skb,
+					   struct net_device *dev,
+					   netdev_features_t features)
+{
+	if (netdev_uses_dsa(dev))
+		features &= ~(NETIF_F_CSUM_MASK | NETIF_F_GSO_MASK);
+
+	return features;
+}
+
 static const struct net_device_ops fe_netdev_ops = {
 	.ndo_init		= fe_init,
 	.ndo_uninit		= fe_uninit,
@@ -1514,6 +1525,7 @@ static const struct net_device_ops fe_netdev_ops = {
 #ifdef CONFIG_NET_POLL_CONTROLLER
 	.ndo_poll_controller	= fe_poll_controller,
 #endif
+	.ndo_features_check	= fe_features_check,
 };
 
 static void fe_reset_pending(struct fe_priv *priv)
-----------------------------[ cut here ]-----------------------------

This is essentially what Florian said ages ago, it just took me a very
long time to process. I guess what hadn't fully clicked in my head is
that the TX checksumming offload being functional is more a matter of
telling the hardware what are the L3 and L4 offsets, and the csum_offset,
rather than it requiring any particular understanding of the DSA header.
In turn, it means that for "nice" Ethernet controller implementations
where that is the case, it would be actively detrimential to add a new
.ndo_get_dsa_features() or something like that - because such a driver
would report that it supports all DSA header-type formats (trailers are
still broken as long as there isn't a csum_end). And keeping that kind
of driver in sync with all DSA protocols that appear will become a
repetitive task.

So crisis averted, I guess?
Thanks a lot to both of you for the patient explanations!
I retract my proposal for a new ndo and also suggest that the DSA master
driver takes care to not leave as zero a TX checksum it can't offload.