Re: [PATCH RFC 2/4] xen: add bitmap to indicate per-domain state changes

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.11.2021 13:46, Juergen Gross wrote:
> On 22.11.21 11:41, Jan Beulich wrote:
>> On 14.09.2021 14:35, Juergen Gross wrote:
>>> Add a bitmap with one bit per possible domid indicating the respective
>>> domain has changed its state (created, deleted, dying, crashed,
>>> shutdown).
>>>
>>> Registering the VIRQ_DOM_EXC event will result in setting the bits for
>>> all existing domains and resetting all other bits.
>>
>> Generally I view VIRQ_DOM_EXC as overly restrictive already - what if both
>> a xenstore domain and a control domain want respective notification? Hence
> 
> The general idea was that in this case the control domain should
> register a related Xenstore watch.
> 
>> similarly I'm not convinced a single, global instance will do here. Which
>> in turn raises the question whether the approach chosen may not take us
>> far enough, and we wouldn't instead want a more precise notification model
>> (i.e. not just "something has changed").
> 
> Yes, that would be the job of Xenstore. It would provide the more
> fine grained watches for that purpose.

And the watch consumer still wouldn't have a way to distinguish two domain
instances using the same ID, would it?

>>> @@ -1141,6 +1161,8 @@ static void complete_domain_destroy(struct rcu_head 
>>> *head)
>>>         xfree(d->vcpu);
>>>   +    set_bit(d->domain_id, dom_state_changed);
>>> +
>>>       _domain_destroy(d);
>>>         send_global_virq(VIRQ_DOM_EXC);
>>
>> Wouldn't this better be in domain_destroy() immediately after the domain
>> has been taken off the list, and hence is no longer "discoverable"?
> 
> In this case the call of send_global_virq() should be moved, too,
> I guess?

Possibly, albeit I'd see this as a separate change to make. It doesn't
seem outright wrong for the vIRQ to be sent late. But I agree with the
idea of keeping sending and bit-setting together (ideally, even if this
was to stay here, the two would better sit truly side by side).

Jan