Re: [PATCH v3 5/5] utils/qemu-sockets: Introduce inet socket options controlling TCP keep-alive

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Apr 11, 2025 at 02:54:29PM +0100, Daniel P. Berrangé wrote:
> On Tue, Apr 08, 2025 at 01:25:04PM +0200, Juraj Marcin wrote:
> > From: Juraj Marcin <jmarcin@redhat.com>
> > 
> > With the default TCP stack configuration, it could be even 2 hours
> > before the connection times out due to the other side not being
> > reachable. However, in some cases, the application needs to be aware of
> > a connection issue much sooner.
> > 
> > This is the case, for example, for postcopy live migration. If there is
> > no traffic from the migration destination guest (server-side) to the
> > migration source guest (client-side), the destination keeps waiting for
> > pages indefinitely and does not switch to the postcopy-paused state.
> > This can happen, for example, if the destination QEMU instance is
> > started with the '-S' command line option and the machine is not started
> > yet, or if the machine is idle and produces no new page faults for
> > not-yet-migrated pages.
> > 
> > This patch introduces new inet socket parameters that control count,
> > idle period, and interval of TCP keep-alive packets before the
> > connection is considered broken. These parameters are available on
> > systems where the respective TCP socket options are defined
> > (TCP_KEEPCNT, TCP_KEEPIDLE, TCP_KEEPINTVL).
> > 
> > The default value for all is 0, which means the system configuration is
> > used.
> > 
> > Signed-off-by: Juraj Marcin <jmarcin@redhat.com>
> > ---
> >  meson.build         |  6 ++++
> >  qapi/sockets.json   | 15 ++++++++
> >  util/qemu-sockets.c | 88 +++++++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 109 insertions(+)
> > 
> > diff --git a/meson.build b/meson.build
> > index 41f68d3806..680f47cf42 100644
> > --- a/meson.build
> > +++ b/meson.build
> > @@ -2734,6 +2734,12 @@ if linux_io_uring.found()
> >    config_host_data.set('HAVE_IO_URING_PREP_WRITEV2',
> >                         cc.has_header_symbol('liburing.h', 'io_uring_prep_writev2'))
> >  endif
> > +config_host_data.set('HAVE_TCP_KEEPCNT',
> > +                     cc.has_header_symbol('netinet/tcp.h', 'TCP_KEEPCN'T))
> > +config_host_data.set('HAVE_TCP_KEEPIDLE',
> > +                     cc.has_header_symbol('netinet/tcp.h', 'TCP_KEEPIDLE'))
> > +config_host_data.set('HAVE_TCP_KEEPINTVL',
> > +                     cc.has_header_symbol('netinet/tcp.h', 'TCP_KEEPINTVL'))
> 
> What platforms are you aware of that do NOT have these
> settings available ? I'm wondering if we can just assume
> they always exist.

macOS appears to have these, except that 'TCP_KEEPIDLE' is under a
differnt name 'TCP_KEEPALIVE':

  https://github.com/apple/darwin-xnu/blob/xnu-4570.1.46/bsd/man/man4/tcp.4#L172

Likewise I see them available in mingw for Wndows builds, with both
names

$ grep -r TCP_KEEP /usr/i686-w64-mingw32/sys-root/mingw/include/
/usr/i686-w64-mingw32/sys-root/mingw/include/ws2ipdef.h:#define TCP_KEEPALIVE 3
/usr/i686-w64-mingw32/sys-root/mingw/include/ws2ipdef.h:#define TCP_KEEPCNT 16
/usr/i686-w64-mingw32/sys-root/mingw/include/ws2ipdef.h:#define TCP_KEEPIDLE TCP_KEEPALIVE
/usr/i686-w64-mingw32/sys-root/mingw/include/ws2ipdef.h:#define TCP_KEEPINTVL 17

but your patch wouldn't enable it because it checks netinet/tcp.h

AFAICT, the only platform that matters to QEMU that seems to miss this
is OpenBSD 

With regards,
Daniel
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