Re: [Xen-devel] [xen-unstable test] 104131: regressions - FAIL

["Xuquan (Quan Xu)" <xuquan8@xxxxxxxxxx>]

On February 13, 2017 4:24 PM, Tian, Kevin wrote:
>> From: Tian, Kevin
>> Sent: Monday, February 13, 2017 4:21 PM
>>
>> > From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> > Sent: Wednesday, February 08, 2017 4:52 PM
>> >
>> > >>> On 08.02.17 at 09:27, <xuquan8@xxxxxxxxxx> wrote:
>> > > Assumed vCPU is in guest_mode..
>> > > When apicv is enabled, hypervisor calls vmx_deliver_posted_intr(),
>> > > then
>> > > __vmx_deliver_posted_interrupt() to deliver interrupt, but no
>> > > vmexit (also no
>> > > vcpu_kick() )..
>> > > In __vmx_deliver_posted_interrupt(), it is __conditional__ to
>> > > deliver posted interrupt. if posted interrupt is not delivered,
>> > > the posted interrupt is pending until next VM entry -- by PIR to vIRR..
>> > >
>> > > one condition is :
>> > > In __vmx_deliver_posted_interrupt(),  ' if (
>> > > !test_and_set_bit(VCPU_KICK_SOFTIRQ, &softirq_pending(cpu))' ..
>> > >
>> > > Specifically, we did verify it by RES interrupt, which is used for
>> > > smp_reschedule_interrupt..
>> > > We even cost more time to deliver RES interrupt than no-apicv in
>average..
>> > >
>> > > If RES interrupt (no. 1) is delivered by posted way (the vcpu is
>> > > still guest_mode).. when tries to deliver next-coming RES
>> > > interrupt (no. 2) by posted way, The next-coming RES interrupt
>> > > (no. 2) is not delivered, as we set the VCPU_KICK_SOFTIRQ bit when
>> > > we deliver RES interrupt (no. 1)..
>> > >
>> > > Then the next-coming RES interrupt (no. 2) is pending until next
>> > > VM entry -- by PIR to vIRR..
>> > >
>> > >
>> > > We can fix it as below(I don't think this is a best one, it is
>> > > better to set the VCPU_KICK_SOFTIRQ bit, but not test it):
>> > >
>> > > --- a/xen/arch/x86/hvm/vmx/vmx.c
>> > > +++ b/xen/arch/x86/hvm/vmx/vmx.c
>> > > @@ -1846,7 +1846,7 @@ static void
>__vmx_deliver_posted_interrupt(struct vcpu *v)
>> > >      {
>> > >          unsigned int cpu = v->processor;
>> > >
>> > > -        if ( !test_and_set_bit(VCPU_KICK_SOFTIRQ,
>&softirq_pending(cpu))
>> > > +        if ( !test_bit(VCPU_KICK_SOFTIRQ, &softirq_pending(cpu))
>> > >               && (cpu != smp_processor_id()) )
>> > >              send_IPI_mask(cpumask_of(cpu), posted_intr_vector);
>> > >      }
>> >
>> > While I don't think I fully understand your description, the line
>> > you change here has always been puzzling me: If we were to raise a
>> > softirq here, we ought to call cpu_raise_softirq() instead of partly
>> > open coding what it does.
>>
>> We require posted_intr_vector for target CPU to ack/deliver virtual
>> interrupt in non-root mode. cpu_raise_softirq uses a different vector,
>> which cannot trigger such effect.
>>
>> > So I think not marking that softirq
>> > pending (but doing this incompletely) is a valid change in any case.
>> > But I'll have to defer to Kevin in the hopes that he fully
>> > understands what you explain above as well as him knowing why this
>> > was a test-and-set here in the first place.
>> >
>>
>> I agree we have a misuse of softirq mechanism here. If guest is
>> already in non-root mode, the 1st posted interrupt will be directly
>> delivered to guest (leaving softirq being set w/o actually incurring a
>> VM-exit - breaking desired softirq behavior). Then further posted
>> interrupts will skip the IPI, stay in PIR and not noted until another
>> VM-exit happens. Looks Quan observes such delay of delivery in his
>> experiments.
>>
>> I'm OK to remove the set here. Actually since it's an optimization for
>> less IPIs, we'd better check softirq_pending(cpu) directly instead of
>> sticking to one bit only.
>>
>
>sent too fast... Quan, can you work out a patch following this suggestion
>and see whether your slow-delivery issue is solved?
>(hope I understand your issue correctly here).
>

Cool, Very Correct!! 
Sure, I will send out a patch in coming days..

Quan










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