Re: [Xen-devel] [PATCH] qemu-xen: make use of xenstore relative paths

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

On 26/09/13 18:46, Anthony PERARD wrote:
> On Wed, Sep 18, 2013 at 09:50:58PM +0200, Roger Pau Monne wrote:
>> Qemu has several hardcoded xenstore paths that are only valid on Dom0.
>> Attempts to launch a Qemu instance (to act as a userspace backend for
>> PV disks) will fail because Qemu is not able to access those paths
>> when running on a domain different than Dom0.
>>
>> Instead make the xenstore paths relative to the domain where Qemu is
>> actually running.
>>
>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
>> Cc: Anthony PERARD <anthony.perard@xxxxxxxxxx>
>> Cc: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> 
> This look fine. One issue with the patch: the file xen_backend.c have
> been moved to hw/xen/xen_backend.c.

Thanks, this is based on the stable Qemu version in Xen tree, I should
have done the change on top of the main qemu.git repo.

> I've also tryied it in a stubdomain, and it does not boot anymore
> because the qemu in the stubdom can not read the state.  I have tried
> again without the change in xen-all.c, and the stubdom does not complain
> anymore. So in the change in xenstore_record_dm_state() needed as well?

Yes, if we run a Qemu instance inside a driver domain it wouldn't make
much sense IMHO to write the state of that Qemu instance on a xenstore
path that belongs to the Dom0, and also we would need to give the driver
domain permissions to write on a xenstore path that's inside the Dom0
xenstore path, which doesn't seem like a good idea.

To make Qemu work on a domain different than Dom0 you will also need the
following patch from my driver domain series:

http://marc.info/?l=xen-devel&m=137993233817018

If not the guest is unable to create the device-model/<domid>/state
xenstore entry. For stubdomains would it be really hard to change the
Dom0 to check for /local/domain/<stubdom_id>/device-model/<domid>/state
instead of /local/domain/0/device-model/<domid>/state?

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