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SubjectRe: [PATCH v6 08/12] Handle async PF in a guest.
  On 10/04/2010 05:56 PM, Gleb Natapov wrote:
> When async PF capability is detected hook up special page fault handler
> that will handle async page fault events and bypass other page faults to
> regular page fault handler. Also add async PF handling to nested SVM
> emulation. Async PF always generates exit to L1 where vcpu thread will
> be scheduled out until page is available.
>

Please separate guest and host changes.

> +void kvm_async_pf_task_wait(u32 token)
> +{
> + u32 key = hash_32(token, KVM_TASK_SLEEP_HASHBITS);
> + struct kvm_task_sleep_head *b =&async_pf_sleepers[key];
> + struct kvm_task_sleep_node n, *e;
> + DEFINE_WAIT(wait);
> +
> + spin_lock(&b->lock);
> + e = _find_apf_task(b, token);
> + if (e) {
> + /* dummy entry exist -> wake up was delivered ahead of PF */
> + hlist_del(&e->link);
> + kfree(e);
> + spin_unlock(&b->lock);
> + return;
> + }
> +
> + n.token = token;
> + n.cpu = smp_processor_id();
> + init_waitqueue_head(&n.wq);
> + hlist_add_head(&n.link,&b->list);
> + spin_unlock(&b->lock);
> +
> + for (;;) {
> + prepare_to_wait(&n.wq,&wait, TASK_UNINTERRUPTIBLE);
> + if (hlist_unhashed(&n.link))
> + break;
> + local_irq_enable();

Suppose we take another apf here. And another, and another (for
different pages, while executing schedule()). What's to prevent kernel
stack overflow?
> + schedule();
> + local_irq_disable();
> + }
> + finish_wait(&n.wq,&wait);
> +
> + return;
> +}
> +EXPORT_SYMBOL_GPL(kvm_async_pf_task_wait);
> +
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.



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