On 07.01.25 16:34, Jan Beulich wrote:
On 07.01.2025 11:17, Juergen Gross wrote:--- a/xen/common/event_channel.c +++ b/xen/common/event_channel.c @@ -120,6 +120,13 @@ static uint8_t get_xen_consumer(xen_event_channel_notification_t fn) /* Get the notification function for a given Xen-bound event channel. */ #define xen_notification_fn(e) (xen_consumers[(e)->xen_consumer-1])+static struct domain *global_virq_handlers[NR_VIRQS] __read_mostly;Nit: While you move this line around, it would be nice if the attribute could then also move to its canonical place (between type and identifier).+static struct domain *get_global_virq_handler(unsigned int virq) +{ + return global_virq_handlers[virq] ?: hardware_domain; +} + static bool virq_is_global(unsigned int virq) { switch ( virq ) @@ -479,8 +486,13 @@ int evtchn_bind_virq(evtchn_bind_virq_t *bind, evtchn_port_t port) */ virq = array_index_nospec(virq, ARRAY_SIZE(v->virq_to_evtchn));- if ( virq_is_global(virq) && (vcpu != 0) )- return -EINVAL; + if ( virq_is_global(virq) ) + { + if ( get_global_virq_handler(virq) != d ) + return -EBUSY;Hmm. While this eliminates the problem for the common, race free case, the handler changing right after the check would still mean the bind would succeed.
Are you fine with me adding a paragraph to the commit message saying that a future patch will handle this case? This future patch is patch 4 of the series, which will need to be modified to check the handling domain inside the event_lock.
Plus this way you're breaking a case that afaict has been working so far: The bind happening before the setting of the handler. With a lot of unrelated if-s and when-s this could e.g. be of interest when considering a re-startable Xenstore domain. The one to take over could start first, obtain state from the original one while that's still active, and be nominated the handler of the global vIRQ only in the last moment.
This is a racy situation, too. If the old domain receives the virq after sending the state, this would need to be handled by transferring the virq information to the new domain, which can result in a never ending story. This is the reason why the domain state bitmap is reset to contain all existing domains to be flagged as "changed", as otherwise a change might get lost. I'd rather be able to handle today's use cases in a sane way than to try handling any weird future use cases which we don't know yet. I think today's behavior is more or less insane and the new behavior is much easier to understand and more intuitive. Juergen
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