Re: [Xen-devel] [PATCH v5] libxl: fix migration of PV and PVH domUs with and without qemu

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, May 14, 2019 at 10:05:58AM +0200, Olaf Hering wrote:
> If a domU has a qemu-xen instance attached, it is required to call qemus
> "xen-save-devices-state" method. Without it, the receiving side of a PV or
> PVH migration may be unable to lock the image:
> 
> xen be: qdisk-51712: xen be: qdisk-51712: error: Failed to get "write" lock
> error: Failed to get "write" lock
> xen be: qdisk-51712: xen be: qdisk-51712: initialise() failed
> initialise() failed
> 
> To fix this bug, libxl__domain_suspend_device_model() and
> libxl__domain_resume_device_model() have to be called not only for HVM,
> but also if the active device_model is QEMU_XEN.
> 
> Unfortunately, libxl__domain_build_info_setdefault() used to hardcode
> b_info->device_model_version to QEMU_XEN if it does not know it any
> better. As a result libxl__device_model_version_running() will return
> incorrect values. This breaks domUs without a device_model.
> libxl__qmp_stop() would wait 10 seconds in qmp_open() for a qemu that
> will never appear. During this long timeframe the domU remains in state
> paused on the sending side. As a result network connections may be
> dropped. Once this bug is fixed as well, by just removing the assumption
> that every domU has a QEMU_XEN, there is no code to actually initialise
> b_info->device_model_version.
> 
> There is a helper function libxl__need_xenpv_qemu(), which is used in
> various places to decide if a device_model has to be spawned. This
> function can not be used as is, just to fill device_model_version,
> because store_libxl_entry() was already called earlier.
> 
> Introduce LIBXL_DEVICE_MODEL_VERSION_NONE for PV and PVH that have no
> need for a device_model to make the state explicit. Indicate this new
> state via LIBXL_HAVE macro in libxl.h.
> 
> v05:
> - move initialization of device_model_version to extra commit
> - return error from libxl__need_xenpv_qemu
> - add LIBXL_HAVE_DEVICE_MODEL_VERSION_NONE
> v04:
> - make sure device_model_stubdomain is initialized
> v03:
> - rearrange code to make sure device_model_version is initialized before
>   store_libxl_entry() is called
> v02:
> - update wording in a comment
> - remove stale goto in domcreate_launch_dm
> - initialize ret in libxl__need_xenpv_qemu
> 
> Signed-off-by: Olaf Hering <olaf@xxxxxxxxx>

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks, Roger.

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