On 16.11.24 12:01, Julien Grall wrote:
Hi Jan & Juergen,On 04/11/2024 09:35, Jan Beulich wrote: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.While in principle I agree that having multiple consumers of VIRQ_DOM_EXC would be great. I have some scalability concern because now we would end up to have to update N bitmap every time. So we would need to put a limit to N. I don't think there is a good limit...
The same applies regarding sending an event. I don't think the additional setting of a bit is adding a relevant amount of processing time. I agree that a limit is hard to find, but it could be rather high.
So overall, I am not entirely convinced it is worth the trouble.
The only real reason I could see would be a setup without Xenstore. Reason is that with Xenstore all interested parties could register a watch event instead of directly consuming the VIRQ_DOM_EXC event. We haven't needed multiple VIRQ_DOM_EXC consumers up to now, so I don't think we should over-engineer the interface. At least there is a theoretical solution for multiple consumers, so my patch series wouldn't introduce a no-go for multiple consumers. Juergen
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