Jens Axboe wrote:
-int xen_smp_call_function_mask(cpumask_t mask, void (*func)(void *), - void *info, int wait)
[...]
- /* Send a message to other CPUs and wait for them to respond */ - xen_send_IPI_mask(mask, XEN_CALL_FUNCTION_VECTOR); - - /* Make sure other vcpus get a chance to run if they need to. */ - yield = false; - for_each_cpu_mask(cpu, mask) - if (xen_vcpu_stolen(cpu)) - yield = true; - - if (yield) - HYPERVISOR_sched_op(SCHEDOP_yield, 0);
I added this to deal with the case where you're sending an IPI to another VCPU which isn't currently running on a real cpu. In this case you could end up spinning while the other VCPU is waiting for a real CPU to run on. (Basically the same problem that spinlocks have in a virtual environment.)
However, this is at best a partial solution to the problem, and I never benchmarked if it really makes a difference. Since any other virtual environment would have the same problem, its best if we can solve it generically. (Of course a synchronous single-target cross-cpu call is a simple cross-cpu rpc, which could be implemented very efficiently in the host/hypervisor by simply doing a vcpu context switch...)
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