[libvirt] [PATCH] libxl: fix disk detach when <driver> not specified

[Jim Fehlig]

On 02/07/2017 08:30 AM, Jim Fehlig wrote:
> On 02/07/2017 03:26 AM, Michal Privoznik wrote:
>> On 02/06/2017 11:08 PM, Jim Fehlig wrote:
>>> When a user does not explicitly set a <driver> in the disk config,
>>> libvirt defers selection of a default to libxl. This approach works
>>> fine when starting a domain with such configuration or attaching a
>>> disk to a running domain. But when detaching such a disk, libxl
>>> will fail with "unrecognized disk backend type: 0". libxl makes no
>>> attempt to recalculate a default backend (driver) on detach and
>>> simply fails when uninitialized.
>>>
>>> This patch updates the libvirt disk config with the backend selected
>>> by libxl when starting a domain or attaching a disk to a running
>>> domain. Another benefit of this approach is that the live XML is
>>> also updated with the backend driver selected by libxl.
>>>
>>> Signed-off-by: Jim Fehlig <jfehlig at suse.com>
>>> ---
>>>  src/libxl/libxl_conf.c   | 33 +++++++++++++++++++++++++++++++++
>>>  src/libxl/libxl_conf.h   |  4 ++++
>>>  src/libxl/libxl_domain.c | 25 +++++++++++++++++++++++++
>>>  src/libxl/libxl_driver.c |  2 +-
>>>  4 files changed, 63 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/src/libxl/libxl_conf.c b/src/libxl/libxl_conf.c
>>> index b5186f2..6ef7570 100644
>>> --- a/src/libxl/libxl_conf.c
>>> +++ b/src/libxl/libxl_conf.c
>>> @@ -851,6 +851,7 @@ libxlMakeDisk(virDomainDiskDefPtr l_disk,
>>> libxl_device_disk *x_disk)
>>>           * xl-disk-configuration.txt in the xen documentation and let
>>>           * libxl pick a suitable backend.
>>>           */
>>> +        virDomainDiskSetFormat(l_disk, VIR_STORAGE_FILE_RAW);
>>>          x_disk->format = LIBXL_DISK_FORMAT_RAW;
>>>          x_disk->backend = LIBXL_DISK_BACKEND_UNKNOWN;
>>
>> This doesn't feel right. I know what do you want it here for, but this
>> function is meant to take 'const' disk definition and translate it to
>> libxl definition. Although, I don't have a better suggestion where to
>> put it.
>
> How about in the UpdateDisk function introduced in this patch? E.g. update it to
> raw if current format is VIR_STORAGE_FILE_NONE?

After thinking about it some more, seems the best place to set the default when 
format is VIR_STORAGE_FILE_NONE is in the device post-parse callback. I've sent 
a V2 along those lines

https://www.redhat.com/archives/libvir-list/2017-February/msg00248.html

Regards,
Jim