[Xen-devel] [RFC XEN PATCH for-4.13 0/4] Fix: libxl workaround, multiple connection to single QMP socket

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

Patch series available in this git branch:
https://xenbits.xen.org/git-http/people/aperard/xen-unstable.git 
br.fix-ev_qmp-multi-connect-v1

Hi,

QEMU's QMP socket doesn't allow multiple concurrent connection. Also, it
listen() on the socket with a `backlog' of only 1. On Linux at least, once that
backlog is filled connect() will return EAGAIN if the socket fd is
non-blocking. libxl may attempt many concurrent connect() attempt if for
example a guest is started with several PCI passthrough devices, and a
connect() failure lead to a failure to start the guest.

Since we can't change the listen()'s `backlog' that QEMU use, we need other
ways to workaround the issue. This patch series introduce a lock to acquire
before attempting to connect() to the QMP socket. Since the lock might be held
for to long, the series also introduce a way to cancel the acquisition of the
lock, this means killing the process that tries to get the lock.

Alternatively to this craziness, it might be possible to increase the `backlog'
value by having libxl opening the QMP socket on behalf of QEMU. But this is
only possible with a recent version of QEMU (2.12 or newer, released in Apr
2018, or qemu-xen-4.12 or newer). It would involve to discover QEMU's
capability before we start the DM, which libxl isn't capable yet.

Cheers,

Anthony PERARD (4):
  libxl: Introduce libxl__ev_child_kill
  libxl: Introduce libxl__ev_qmplock
  libxl: libxl__ev_qmp_send now takes an egc
  libxl_qmp: Have a lock for QMP socket access

 tools/libxl/libxl_disk.c        |  6 +--
 tools/libxl/libxl_dm.c          |  8 ++--
 tools/libxl/libxl_dom_save.c    |  2 +-
 tools/libxl/libxl_dom_suspend.c |  2 +-
 tools/libxl/libxl_domain.c      |  8 ++--
 tools/libxl/libxl_event.c       |  3 +-
 tools/libxl/libxl_fork.c        | 55 ++++++++++++++++++++++++
 tools/libxl/libxl_internal.c    | 31 +++++++++++++-
 tools/libxl/libxl_internal.h    | 53 +++++++++++++++++------
 tools/libxl/libxl_pci.c         |  8 ++--
 tools/libxl/libxl_qmp.c         | 75 +++++++++++++++++++++++++--------
 tools/libxl/libxl_usb.c         | 28 ++++++------
 12 files changed, 219 insertions(+), 60 deletions(-)

-- 
Anthony PERARD

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel