Re: [PATCH v2] tcp: probe for SO_PEEK_OFF both in tcpv4 and tcp6

[Stefano Brivio <sbrivio@redhat.com>]

On Tue, 23 Jul 2024 00:09:37 +0200
Stefano Brivio <sbrivio@redhat.com> wrote:

> From: Jon Maloy <jmaloy@redhat.com>
> 
> Based on an original patch by Jon Maloy:
> 
> --
> The recently added socket option SO_PEEK_OFF is not supported for
> TCP/IPv6 sockets. Until we get that support into the kernel we need to
> test for support in both protocols to set the global 'peek_offset_cap´
> to true.
> --
> 
> Compared to the original patch:
> - only check for SO_PEEK_OFF support for enabled IP versions
> - use sa_family_t instead of int to pass the address family around
> 
> Fixes: e63d281871ef ("tcp: leverage support of SO_PEEK_OFF socket option when available")

...so, with this, the probing issue is solved: on a 6.10 kernel,
SO_PEEK_OFF is not used, unless I disable IPv6 (with --ipv4-only / -4).

However, if I disable it, for some reason, resorting to IPv4, at least
together with the flow table (applying just this patch to HEAD), I get
something that looks like one of the "old" TCP stalls. On the host:

  $ ./passt -f -t 10000 -4

and in the guest:

  # ip link set dev eth0 up
  # dhclient eth0
  # iperf3 -s -p 10000

back to the host:

  $ iperf3 -c 127.0.0.1 -p 10000
  Connecting to host 127.0.0.1, port 10000
  [  5] local 127.0.0.1 port 39046 connected to 127.0.0.1 port 10000
  [ ID] Interval           Transfer     Bitrate         Retr  Cwnd
  [  5]   0.00-1.00   sec  11.2 MBytes  94.3 Mbits/sec    0   5.50 MBytes       
  [  5]   1.00-2.00   sec  0.00 Bytes  0.00 bits/sec    0   5.50 MBytes       
  [  5]   2.00-3.00   sec  0.00 Bytes  0.00 bits/sec    0   5.50 MBytes       

...the transfer never recovers.

I didn't really have time to debug this further.

At the moment I would be inclined to temporarily revert commit
e63d281871ef ("tcp: leverage support of SO_PEEK_OFF socket option when
available"), but it's not a good idea if this happens to be hiding some
(unlikely?) issue with the flow table.

-- 
Stefano