Re: [PATCH 2/6] xen: add bitmap to indicate per-domain state changes

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.11.2024 07:48, Jürgen Groß wrote:
> On 31.10.24 11:59, Jan Beulich wrote:
>> On 23.10.2024 15:10, 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.
>>
>> That's furthering the "there can be only one consumer" model that also
>> is used for VIRQ_DOM_EXC itself. I consider the existing model flawed
>> (nothing keeps a 2nd party with sufficient privilege from invoking
>> XEN_DOMCTL_set_virq_handler a 2nd time, taking away the notification
>> from whoever had first requested it), and hence I dislike this being
>> extended. Conceivably multiple parties may indeed be interested in
>> this kind of information. At which point resetting state when the vIRQ
>> is bound is questionable (or the data would need to become per-domain
>> rather than global, or even yet more fine-grained, albeit
>> ->virq_to_evtchn[] is also per-domain, when considering global vIRQ-s).
> 
> The bitmap is directly tied to the VIRQ_DOM_EXC anyway, as it is that
> event which makes the consumer look into the bitmap via the new hypercall.
> 
> If we decide to allow multiple consumers of VIRQ_DOM_EXC, we'll need to
> have one bitmap per consumer of the event. This is not very hard to
> modify.
> 
> If you'd like that better, I can dynamically allocate the bitmap on
> binding VIRQ_DOM_EXC and freeing it again when unbinding is done.

I'd prefer that indeed, yet I'm also curious what other maintainers think.

Jan