Re: [PATCH] docs: add misc/qemu-backends.txt

[Juergen Gross <jgross@suse.com>]

On 10/04/16 22:00, Stefano Stabellini wrote:
> On Thu, 7 Apr 2016, Juergen Gross wrote:
>> Document the interface between qemu and libxl regarding backends
>> supported by qemu.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>>  docs/misc/qemu-backends.txt | 19 +++++++++++++++++++
>>  1 file changed, 19 insertions(+)
>>  create mode 100644 docs/misc/qemu-backends.txt
>>
>> diff --git a/docs/misc/qemu-backends.txt b/docs/misc/qemu-backends.txt
>> new file mode 100644
>> index 0000000..f28755e
>> --- /dev/null
>> +++ b/docs/misc/qemu-backends.txt
>> @@ -0,0 +1,19 @@
>> +In order to know whether qemu supports a specific backend type libxl
>> +needs a way to obtain this information.
>> +
>> +As each qemu instance owns a path (named "<qemu>" from now on) in
>> +Xenstore the backend information is presented there. <qemu> is built
>> +from the domain id where the qemu instance is running <backend-dom>
>> +and the domain id of the target domain of the qemu process <domid>:
>> +
>> +<qemu> = /local/domain/<backend-dom>/device-model/<domid>
>> +
>> +Before signalling qemu is running by writing "running" to <qemu>/state
>> +qemu will create a Xenstore node for each supported backend under
>> +<qemu>/backends with the backend type as name (e.g.
>> +<qemu>/backends/qdisk for the qdisk backend).
>> +
>> +libxl can assume a backend of a specific type <type> is supported if:
>> +- <qemu>/backends/<type> is existing in Xenstore
>> +- or <qemu>/backends is not existing and <type> is one of:
>> +  "console", "vkbd", "vfb", "qdisk", "qnic"
> 
> The thing to be careful about is that the plan just a few months ago was
> to have QEMU restrict its own xenstore connection to the privilege level
> of the guest VM it was servicing. Libxl would relax the xenstore access
> rights to allow QEMU (and the gueest VM) access to
> /local/domain/<backend-dom>/device-model/<domid>/physmap, but nothing
> else. See:
> 
> [1] http://marc.info/?l=qemu-devel&m=143317363104584&w=2
> [2] http://marc.info/?l=xen-devel&m=145081000327541
> 
> what that means is that QEMU wouldn't be able to write to
> /local/domain/<backend-dom>/device-model/<domid>/backends, unless the
> writing was done before calling xsrestrict, which should be
> doable, but not what was done in [1].
> 
> Maybe we could add a note saying that these paths need to be written by
> QEMU before dropping xenstore privileges?

Okay.


Juergen


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