Am 25.11.2014 um 17:04 schrieb David Hildenbrand:
> As some architectures (e.g. s390) can't disable preemption while
> entering/leaving the guest, they won't receive the yield in all situations.
>
> kvm_enter_guest() has to be called with preemption_disabled and will set
> PF_VCPU. After that point e.g. s390 reenables preemption and starts to
> execute the
> guest. The thread might therefore be scheduled out between kvm_enter_guest()
> and
> kvm_exit_guest(), resulting in PF_VCPU being set but not being run.
>
> Please note that preemption has to stay enabled in order to correctly process
> page faults on s390.
>
> Current code takes PF_VCPU as a hint that the VCPU thread is running and
> therefore needs no yield. yield_to() checks whether the target thread is
> running,
> so let's use the inbuilt functionality to make it independent of PF_VCPU and
> preemption.
This change is a trade-off.
PRO: This patch would improve the case of preemption on s390. This is probably
a corner case as most distros have preemption off anyway.
CON: The downside is that kvm_vcpu_yield_to is called also from
kvm_vcpu_on_spin. Here we want to avoid the scheduler overhead for a wrong
decision.
So I think this patch is probably not what we want in most cases.
>
> Signed-off-by: David Hildenbrand <d...@linux.vnet.ibm.com>
> ---
> virt/kvm/kvm_main.c | 4 ----
> 1 file changed, 4 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 5b45330..184f52e 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1782,10 +1782,6 @@ int kvm_vcpu_yield_to(struct kvm_vcpu *target)
> rcu_read_unlock();
> if (!task)
> return ret;
> - if (task->flags & PF_VCPU) {
> - put_task_struct(task);
> - return ret;
> - }
> ret = yield_to(task, 1);
> put_task_struct(task);
>
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