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

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
  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

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:


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?

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


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