Re: [PATCH v8 7/8] vhost-user: add vhost-user

On Thu, 17 Oct 2024 02:10:31 +0200 Stefano Brivio wrote:

On Wed, 16 Oct 2024 11:41:34 +1100 David Gibson wrote:

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On Tue, Oct 15, 2024 at 09:54:38PM +0200, Stefano Brivio wrote:

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[Still partial review] [snip]

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+ if (peek_offset_cap) + already_sent = 0; + + iov_vu[0].iov_base = tcp_buf_discard; + iov_vu[0].iov_len = already_sent;

I think I had a similar comment to a previous revision. Now, I haven't tested this (yet) on a kernel with support for SO_PEEK_OFF on TCP, but I think this should eventually follow the same logic as the (updated) tcp_buf_data_from_sock(): we should use tcp_buf_discard only if (!peek_offset_cap).

It's fine to always initialise VIRTQUEUE_MAX_SIZE iov_vu items, starting from 1, for simplicity. But I'm not sure if it's safe to pass a zero iov_len if (peek_offset_cap).

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I'll test that (unless you already did) -- if it works, we can fix this up later as well.

I believe I tested it at some point, and I think we're already using it somewhere.

I tested it again just to be sure on a recent net.git kernel: sometimes the first test in passt_vu_in_ns/tcp, "TCP/IPv4: host to guest: big transfer" hangs on my setup, sometimes it's the "TCP/IPv4: ns to guest (using loopback address): big transfer" test instead.

I can reproduce at least one of the two issues consistently (tests stopped 5 times out of 5).

The socat client completes the transfer, the server is still waiting for something. I haven't taken captures yet or tried to re-send from the client.

...Laurent, let me know if I should dig into this any further. For reference, the kernel commit introducing SO_PEEK_OFF support for TCP on IPv6 is be9a4fb831b8 ("tcp: add SO_PEEK_OFF socket option tor TCPv6"). Without that commit, passt won't set peek_offset_cap. It was added in 6.11-rc5, so it's part of kernel-6.11.3-200.fc40 (latest stable kernel) for Fedora 40. passt will print "SO_PEEK_OFF supported" if you run it with -d -f. -- Stefano