On Thu, Oct 07, 2010 at 07:18:03PM +0200, Avi Kivity wrote:
> On 10/07/2010 07:14 PM, Gleb Natapov wrote:
> >On Thu, Oct 07, 2010 at 03:10:27PM +0200, Avi Kivity wrote:
> >> 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?
> >>
> >Host side keeps track of outstanding apfs and will not send apf for the
> >same phys address twice. It will halt vcpu instead.
>
> What about different pages, running the scheduler code?
>
We can get couple of nested apfs, just like we can get nested
interrupts. Since scheduler disables preemption second apf will halt.
> Oh, and we'll run the scheduler recursively.
>
As rick said scheduler disables preemption. And this is actually first
thing it does. Otherwise any interrupt may cause recursive scheduler
invocation.
--
Gleb.
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