Re: [PATCH 2/3] Mini-OS: don't crash if no shutdown node is available

[Juergen Gross <jgross@suse.com>]

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On 01.11.23 17:57, Andrew Cooper wrote:
> On 01/11/2023 4:42 pm, Juergen Gross wrote:
>> On 01.11.23 17:38, Andrew Cooper wrote:
>>> On 01/11/2023 9:00 am, Juergen Gross wrote:
>>>> It might be perfectly fine not to have a control/shutdown Xenstore
>>>> node. If this is the case, don't crash, but just terminate the
>>>> shutdown thread after issuing a message that shutdown isn't available.
>>>>
>>>> In fini_shutdown() clearing the watch can result in an error now, in
>>>> case the early exit above was taken. Just ignore this error now.
>>>>
>>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>>
>>> Which cases might we not have a control/shutdown node?
>>
>> Xenstore-stubdom. It should _never_ shutdown, and it isn't really under
>> control of Xen tools (other than being created).
>>
>>> I'm all for coping better with its absence, but it's not a piece of the
>>> Xen ABI which is optional.
>>
>> I'd like to differ here. See reasoning above.
> 
> If we're going to permit this configuration, then I think it needs an
> extension to xenstore-paths to make it officially optional.
> 
> And I think it's reasonable to support, but I wouldn't go as far as
> saying "never".  If you've cleaved the global xenstored in
> twain/trine/etc, then individual parts of it can shut down normally.

Xenstore-stubdom is a very special case. I don't think its shutdown node
can be under control of the normal Xen tools, as only the stubdom can know
whether it is able to react in any sensible way to it. It needs to take
specific measures to ensure that even its ABI-compliant reaction to a
shutdown request is visible to Xen tools (remember that this reaction is
a write to the shutdown node causing a watch event in dom0, which won't
work with Xenstore going down).

In the end there is no way the shutdown node can be present when starting
Xenstore-stubdom. There is no Xenstore at the time the node is probed in
today's Mini-OS boot sequence.


Juergen

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