Re: [PATCH v6 02/12] Halt vcpu if page it tries to access is swappedout.

From: Avi Kivity
Date: Sun Oct 10 2010 - 11:56:05 EST

On 10/10/2010 09:29 AM, Gleb Natapov wrote:
On Sat, Oct 09, 2010 at 08:30:18PM +0200, Avi Kivity wrote:
> On 10/07/2010 07:47 PM, Gleb Natapov wrote:
> >On Thu, Oct 07, 2010 at 11:50:08AM +0200, Avi Kivity wrote:
> >> On 10/04/2010 05:56 PM, Gleb Natapov wrote:
> >> >If a guest accesses swapped out memory do not swap it in from vcpu thread
> >> >context. Schedule work to do swapping and put vcpu into halted state
> >> >instead.
> >> >
> >> >Interrupts will still be delivered to the guest and if interrupt will
> >> >cause reschedule guest will continue to run another task.
> >> >
> >> >
> >> >+
> >> >+static bool can_do_async_pf(struct kvm_vcpu *vcpu)
> >> >+{
> >> >+ if (unlikely(!irqchip_in_kernel(vcpu->kvm) ||
> >> >+ kvm_event_needs_reinjection(vcpu)))
> >> >+ return false;
> >> >+
> >> >+ return kvm_x86_ops->interrupt_allowed(vcpu);
> >> >+}
> >>
> >> Strictly speaking, if the cpu can handle NMIs it can take an apf?
> >>
> >We can always do apf, but if vcpu can't do anything hwy bother. For NMI
> >watchdog yes, may be it is worth to allow apf if nmi is allowed.
>
> Actually it's very dangerous - the IRET from APF will re-enable
> NMIs. So without the guest enabling apf-in-nmi we shouldn't allow
> it.
>
Good point.

> Not worth the complexity IMO.
>
> >> >@@ -5112,6 +5122,13 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> >> > if (unlikely(r))
> >> > goto out;
> >> >
> >> >+ kvm_check_async_pf_completion(vcpu);
> >> >+ if (vcpu->arch.mp_state == KVM_MP_STATE_HALTED) {
> >> >+ /* Page is swapped out. Do synthetic halt */
> >> >+ r = 1;
> >> >+ goto out;
> >> >+ }
> >> >+
> >>
> >> Why do it here in the fast path? Can't you halt the cpu when
> >> starting the page fault?
> >Page fault may complete before guest re-entry. We do not want to halt vcpu
> >in this case.
>
> So unhalt on completion.
>
I want to avoid touching vcpu state from work if possible. Work code does
not contain arch dependent code right now and mp_state is x86 thing


Use a KVM_REQ.


> >>
> >> I guess the apf threads can't touch mp_state, but they can have a
> >> KVM_REQ to trigger the check.
> >This will require KVM_REQ check on fast path, so what's the difference
> >performance wise.
>
> We already have a KVM_REQ check (if (vcpu->requests)) so it doesn't
> cost anything extra.
if (vcpu->requests) does not clear req bit, so what will have to be added
is: if (kvm_check_request(KVM_REQ_APF_HLT, vcpu)) which is even more
expensive then my check (but not so expensive to worry about).

It's only expensive when it happens. Most entries will have the bit clear.

>
> >> >
> >> >@@ -6040,6 +6064,7 @@ void kvm_arch_flush_shadow(struct kvm *kvm)
> >> > int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
> >> > {
> >> > return vcpu->arch.mp_state == KVM_MP_STATE_RUNNABLE
> >> >+ || !list_empty_careful(&vcpu->async_pf.done)
> >> > || vcpu->arch.mp_state == KVM_MP_STATE_SIPI_RECEIVED
> >> > || vcpu->arch.nmi_pending ||
> >> > (kvm_arch_interrupt_allowed(vcpu)&&
> >>
> >> Unrelated, shouldn't kvm_arch_vcpu_runnable() look at
> >> vcpu->requests? Specifically KVM_REQ_EVENT?
> >I think KVM_REQ_EVENT is covered by checking nmi and interrupt queue
> >here.
>
> No, the nmi and interrupt queues are only filled when the lapic is
> polled via KVM_REQ_EVENT. I'll prepare a patch.
I don't think you are correct. nmi_pending is filled before setting
KVM_REQ_EVENT and kvm_cpu_has_interrupt() checks directly in apic/pic.

Right.

>
> >> >+
> >> >+TRACE_EVENT(
> >> >+ kvm_async_pf_not_present,
> >> >+ TP_PROTO(u64 gva),
> >> >+ TP_ARGS(gva),
> >>
> >> Do you actually have a gva with tdp? With nested virtualization,
> >> how do you interpret this gva?
> >With tdp it is gpa just like tdp_page_fault gets gpa where shadow page
> >version gets gva. Nested virtualization is too complex to interpret.
>
> It's not good to have a tracepoint that depends on cpu mode (without
> recording that mode). I think we have the same issue in
> trace_kvm_page_fault though.
We have mmu_is_nested(). I'll just disable apf while vcpu is in nested
mode for now.

What if we get the apf in non-nested mode and it completes in nested mode?

>
> >> >+
> >> >+ /* do alloc nowait since if we are going to sleep anyway we
> >> >+ may as well sleep faulting in page */
> >> /*
> >> * multi
> >> * line
> >> * comment
> >> */
> >>
> >> (but a good one, this is subtle)
> >>
> >> I missed where you halt the vcpu. Can you point me at the function?
> >>
> >> Note this is a synthetic halt and must not be visible to live
> >> migration, or we risk live migrating a halted state which doesn't
> >> really exist.
> >>
> >> Might be simplest to drain the apf queue on any of the save/restore ioctls.
> >>
> >So that "info cpu" will interfere with apf? Migration should work
> >in regular way. apf state should not be migrated since it has no meaning
> >on the destination. I'll make sure synthetic halt state will not
> >interfere with migration.
>
> If you deliver an apf, the guest expects a completion.
>
There is special completion that tells guest to wake all sleeping tasks
on vcpu. It is delivered after migration on the destination.


Yes, I saw.

What if you can't deliver it? is it possible that some other vcpu will start receiving apfs that alias the old ones? Or is the broadcast global?

--
error compiling committee.c: too many arguments to function

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/