[Xen-devel] [XEN PATCH for-4.13 v2 0/6] 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-v2

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.

See thread[1] for discussed alternative.
[1] https://lists.xenproject.org/archives/html/xen-devel/2019-10/msg01815.html

Cheers,

Anthony PERARD (6):
  libxl: Introduce libxl__ev_child_kill_deregister
  libxl: Move libxl__ev_devlock declaration
  libxl: Rename ev_devlock to ev_slowlock
  libxl: Introduce libxl__ev_slowlock_dispose
  libxl: libxl__ev_qmp_send now takes an egc
  libxl_qmp: Have a lock for QMP socket access

 tools/libxl/libxl_disk.c        |  16 ++--
 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      |  18 ++---
 tools/libxl/libxl_event.c       |   6 +-
 tools/libxl/libxl_fork.c        |  48 ++++++++++++
 tools/libxl/libxl_internal.c    |  41 +++++++---
 tools/libxl/libxl_internal.h    | 130 +++++++++++++++++++-------------
 tools/libxl/libxl_pci.c         |   8 +-
 tools/libxl/libxl_qmp.c         | 119 ++++++++++++++++++++++++-----
 tools/libxl/libxl_usb.c         |  28 ++++---
 12 files changed, 301 insertions(+), 125 deletions(-)

-- 
Anthony PERARD


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