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SubjectRe: [PATCH 2/10] x86: convert to generic helpers for IPI function calls
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...)

J


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