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

[Sander Eikelenboom <sander@xxxxxxxxxxxxxx>]

On 25/10/2019 19:05, Anthony PERARD wrote:
> 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.

Hi Anthony,

Just tested with the patch series and it fixes my issue with starting a
guest with several PCI passthrough devices.

Thanks,

Sander


> 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(-)
> 


-- 

Met vriendelijke groet,

Sander Eikelenboom
mailto:Sander@xxxxxxxxxxxxxx

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