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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2013-04-26 at 12:07 +0100, Dave Scott wrote:
> 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@xxxxxxxxxxxxx>
> >>>> Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> >>>
> >>> Acked-by: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
> >>
> >> 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?

That's my working theory at the minute.

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

I'm not sure why we can't do the libxl__try_phy_backend at the same time
as the stat, all it does is abstract the difference away between Linux
and BSD duplicating a subset of the existing test after the stat...

Ian.


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel