Re: [PATCH v2] Only call stat() when adding a disk if we expect a device to exist.

[David Scott <dave.scott@eu.citrix.com>]

On 26/04/13 11:55, Ian Campbell wrote:
> On Wed, 2013-04-24 at 12:56 +0100, Ian Campbell wrote:
>> On Tue, 2013-04-23 at 15:08 +0100, Roger Pau Monne wrote:
>>> On 23/04/13 11:59, David Scott wrote:
>>>> We consider calling stat() a helpful error check in the following
>>>> circumstances only:
>>>>   1. the disk backend type must be PHYsical
>>>>   2. the disk backend domain must be the same as the running libxl
>>>>      code (ie LIBXL_TOOLSTACK_DOMID)
>>>>   3. there must not be a hotplug script because this would imply that
>>>>      the device won't be created until after the hotplug script has
>>>>      run.
>>>>
>>>> With this fix, it is possible to use qemu's built-in block drivers
>>>> such as ceph/rbd, with a xl config disk spec like this:
>>>>
>>>> disk=[ 'backendtype=qdisk,format=raw,vdev=hda,access=rw,target=rbd:rbd/ubuntu1204.img' ]
>>>>
>>>> Signed-off-by: David Scott <dave.scott@eu.citrix.com>
>>>> Acked-by: Ian Campbell <ian.campbell@citrix.com>
>>>
>>> Acked-by: Roger Pau Monné <roger.pau@citrix.com>
>>
>> Applied, thanks.
>
> This patch had the slightly unfortunate impact of causing libxl to not
> use blkback for actual block devices present in dom0, causing it to fall
> back to tap/qemu.
>
> I'm having a look now...

Thanks (and sorry!)

Looking at the code again, is it because bypassing the stat() in the 
UNKNOWN case where in fact it is a block device, leaves a.stab 
uninitialised which then confuses

         if (libxl__try_phy_backend(a->stab.st_mode))
             return backend;

in disk_try_backend?

Perhaps a better patch would do the stat() anyway and only worry about 
the failure if (disk->backend = ... PHY && disk->backend_domid = ... etc)


Cheers,
Dave

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