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

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 16 Oct 2024 11:41:34 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Tue, Oct 15, 2024 at 09:54:38PM +0200, Stefano Brivio wrote:
> > [Still partial review]  
> [snip]
> > > +	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).  
> 
> > 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.

It all works (consistently) with an older kernel without support for
SO_PEEK_OFF on TCP, but also on this kernel if I force peek_offset_cap
to false in tcp_init().

-- 
Stefano