Re: [RFC PATCH 5/5] sockets: Support multipath TCP

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Daniel P. Berrangé (berrange@redhat.com) wrote:
> On Thu, Apr 08, 2021 at 08:11:59PM +0100, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > Multipath TCP allows combining multiple interfaces/routes into a single
> > socket, with very little work for the user/admin.
> > 
> > It's enabled by 'mptcp' on most socket addresses:
> > 
> >    ./qemu-system-x86_64 -nographic -incoming tcp:0:4444,mptcp
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> >  io/dns-resolver.c   |  2 ++
> >  qapi/sockets.json   |  5 ++++-
> >  util/qemu-sockets.c | 34 ++++++++++++++++++++++++++++++++++
> >  3 files changed, 40 insertions(+), 1 deletion(-)
> > 
> > diff --git a/io/dns-resolver.c b/io/dns-resolver.c
> > index 743a0efc87..b081e098bb 100644
> > --- a/io/dns-resolver.c
> > +++ b/io/dns-resolver.c
> > @@ -122,6 +122,8 @@ static int qio_dns_resolver_lookup_sync_inet(QIODNSResolver *resolver,
> >              .ipv4 = iaddr->ipv4,
> >              .has_ipv6 = iaddr->has_ipv6,
> >              .ipv6 = iaddr->ipv6,
> > +            .has_mptcp = iaddr->has_mptcp,
> > +            .mptcp = iaddr->mptcp,
> >          };
> >  
> >          (*addrs)[i] = newaddr;
> > diff --git a/qapi/sockets.json b/qapi/sockets.json
> > index 2e83452797..43122a38bf 100644
> > --- a/qapi/sockets.json
> > +++ b/qapi/sockets.json
> > @@ -57,6 +57,8 @@
> >  # @keep-alive: enable keep-alive when connecting to this socket. Not supported
> >  #              for passive sockets. (Since 4.2)
> >  #
> > +# @mptcp: enable multi-path TCP. (Since 6.0)
> > +#
> >  # Since: 1.3
> >  ##
> >  { 'struct': 'InetSocketAddress',
> > @@ -66,7 +68,8 @@
> >      '*to': 'uint16',
> >      '*ipv4': 'bool',
> >      '*ipv6': 'bool',
> > -    '*keep-alive': 'bool' } }
> > +    '*keep-alive': 'bool',
> > +    '*mptcp': 'bool' } }
> 
> I think this would need to be
> 
>    '*mptcp': { 'type': 'bool', 'if': 'IPPROTO_MPTCP' }
> 
> so that mgmt apps can probe when it truely is supported or not for
> this build

Done; now remember that just tells you if your C library knows about it,
not whether your kernel, firewall, or avian carriers know about it.

> >  
> >  ##
> >  # @UnixSocketAddress:
> > diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c
> > index 8af0278f15..72527972d5 100644
> > --- a/util/qemu-sockets.c
> > +++ b/util/qemu-sockets.c
> > @@ -206,6 +206,21 @@ static int try_bind(int socket, InetSocketAddress *saddr, struct addrinfo *e)
> >  #endif
> >  }
> >  
> > +static int check_mptcp(const InetSocketAddress *saddr, struct addrinfo *ai,
> > +                       Error **errp)
> > +{
> > +    if (saddr->has_mptcp && saddr->mptcp) {
> > +#ifdef IPPROTO_MPTCP
> > +        ai->ai_protocol = IPPROTO_MPTCP;
> > +#else
> > +        error_setg(errp, "MPTCP unavailable in this build");
> > +        return -1;
> > +#endif
> > +    }
> > +
> > +    return 0;
> > +}
> > +
> >  static int inet_listen_saddr(InetSocketAddress *saddr,
> >                               int port_offset,
> >                               int num,
> > @@ -278,6 +293,11 @@ static int inet_listen_saddr(InetSocketAddress *saddr,
> >  
> >      /* create socket + bind/listen */
> >      for (e = res; e != NULL; e = e->ai_next) {
> > +        if (check_mptcp(saddr, e, &err)) {
> > +            error_propagate(errp, err);
> > +            return -1;
> > +        }
> 
> So this is doing two different things - it checks whether mptcp was
> requested and if not compiled in, reports an error. Second it sets
> the mptcp flag. The second thing is suprising given the name of
> the function but also it delays error reporting until after we've
> gone through the DNS lookup which I think is undesirable.
> 
> If we make the 'mptcp' field in QAPI schema use the conditional that
> I show above, then we make it literally impossible to have the mptcp
> field set when IPPROTO_MPTCP is unset, avoiding the need to do error
> reporting at all.
> 
> IOW, the above 4 lines could be simplified to just
> 
>  #ifdef IPPROTO_MPTCP
>     if (saddr->has_mptcp && saddr->mptcp) {
>         ai->ai_protocol = IPPROTO_MPTCP;
>     }
>  #else

OK, done - (with a #endif)

> 
> 
> > @@ -687,6 +712,15 @@ int inet_parse(InetSocketAddress *addr, const char *str, Error **errp)
> >          }
> >          addr->has_keep_alive = true;
> >      }
> > +    begin = strstr(optstr, ",mptcp");
> > +    if (begin) {
> > +        if (inet_parse_flag("mptcp", begin + strlen(",mptcp"),
> > +                            &addr->mptcp, errp) < 0)
> > +        {
> > +            return -1;
> > +        }
> > +        addr->has_mptcp = true;
> > +    }
> 
> This reminds me that inet_parse_flag is a bit of a crude design right
> now, because it only does half the job, leaving half the repeated code
> pattern in the caller still, with use having the string ",mtcp" /"mptcp"
> repeated three times !

Yeh I noticed that.

> If you fancy refactoring it, i think it'd make more sense if we could
> just have a caller pattern of
> 
>    if (inet_parse_flag(optstr,
>                        "mptcp",
>                        &addr->has_mptcp,
>                        &addr->mptcp, errp) < 0)
> 
> Not a blocker todo this though.

A job for another day.

Dave

> 
> Regards,
> Daniel
> -- 
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-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK