RE: [v3 24/26] KVM: Update Posted-Interrupts Descriptor when vCPU is blocked

[Wu, Feng]

> -----Original Message-----
> From: Marcelo Tosatti [mailto:mtosatti@xxxxxxxxxx]
> Sent: Tuesday, March 31, 2015 7:56 AM
> To: Wu, Feng
> Cc: hpa@xxxxxxxxx; tglx@xxxxxxxxxxxxx; mingo@xxxxxxxxxx; x86@xxxxxxxxxx;
> gleb@xxxxxxxxxx; pbonzini@xxxxxxxxxx; dwmw2@xxxxxxxxxxxxx;
> joro@xxxxxxxxxx; alex.williamson@xxxxxxxxxx; jiang.liu@xxxxxxxxxxxxxxx;
> eric.auger@xxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx
> Subject: Re: [v3 24/26] KVM: Update Posted-Interrupts Descriptor when vCPU is
> blocked
> 
> On Mon, Mar 30, 2015 at 04:46:55AM +0000, Wu, Feng wrote:
> >
> >
> > > -----Original Message-----
> > > From: Marcelo Tosatti [mailto:mtosatti@xxxxxxxxxx]
> > > Sent: Saturday, March 28, 2015 3:30 AM
> > > To: Wu, Feng
> > > Cc: hpa@xxxxxxxxx; tglx@xxxxxxxxxxxxx; mingo@xxxxxxxxxx;
> x86@xxxxxxxxxx;
> > > gleb@xxxxxxxxxx; pbonzini@xxxxxxxxxx; dwmw2@xxxxxxxxxxxxx;
> > > joro@xxxxxxxxxx; alex.williamson@xxxxxxxxxx; jiang.liu@xxxxxxxxxxxxxxx;
> > > eric.auger@xxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > > iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx
> > > Subject: Re: [v3 24/26] KVM: Update Posted-Interrupts Descriptor when
> vCPU
> > > is blocked
> > >
> > > On Fri, Mar 27, 2015 at 06:34:14AM +0000, Wu, Feng wrote:
> > > > > > Currently, the following code is executed before local_irq_disable() is
> > > called,
> > > > > > so do you mean 1)moving local_irq_disable() to the place before it. 2)
> after
> > > > > interrupt
> > > > > > is disabled, set KVM_REQ_EVENT in case the ON bit is set?
> > > > >
> > > > > 2) after interrupt is disabled, set KVM_REQ_EVENT in case the ON bit
> > > > > is set.
> > > >
> > > > Here is my understanding about your comments here:
> > > > - Disable interrupts
> > > > - Check 'ON'
> > > > - Set KVM_REQ_EVENT if 'ON' is set
> > > >
> > > > Then we can put the above code inside " if
> > > (kvm_check_request(KVM_REQ_EVENT, vcpu) || req_int_win) "
> > > > just like it used to be. However, I still have some questions about this
> > > comment:
> > > >
> > > > 1. Where should I set KVM_REQ_EVENT? In function vcpu_enter_guest(),
> or
> > > other places?
> > >
> > > See below:
> > >
> > > > If in vcpu_enter_guest(), since currently local_irq_disable() is called after
> > > 'KVM_REQ_EVENT'
> > > > is checked, is it helpful to set KVM_REQ_EVENT after local_irq_disable() is
> > > called?
> > >
> > >         local_irq_disable();
> > >
> > > 	*** add code here ***
> >
> > So we need add code like the following here, right?
> >
> >           if ('ON' is set)
> >               kvm_make_request(KVM_REQ_EVENT, vcpu);
> 
> Yes.
> 
> > >         if (vcpu->mode == EXITING_GUEST_MODE || vcpu->requests
> > > 						^^^^^^^^^^^^^^
> 
> Point *1.
> 
> > >             || need_resched() || signal_pending(current)) {
> > >                 vcpu->mode = OUTSIDE_GUEST_MODE;
> > >                 smp_wmb();
> > >                 local_irq_enable();
> > >                 preempt_enable();
> > >                 vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
> > >                 r = 1;
> > >                 goto cancel_injection;
> > >         }
> > >
> > > > 2. 'ON' is set by VT-d hardware, it can be set even when interrupt is
> disabled
> > > (the related bit in PIR is also set).
> > >
> > > Yes, we are checking if the HW has set an interrupt in PIR while
> > > outside VM (which requires PIR->VIRR transfer by software).
> > >
> > > If the interrupt it set by hardware after local_irq_disable(),
> > > VMX-entry will handle the interrupt and perform the PIR->VIRR
> > > transfer and reevaluate interrupts, injecting to guest
> > > if necessary, is that correct ?
> > >
> > > > So does it make sense to check 'ON' and set KVM_REQ_EVENT accordingly
> > > after interrupt is disabled?
> > >
> > > To replace the costly
> > >
> > > +            */
> > > +           if (kvm_x86_ops->hwapic_irr_update)
> > > +                   kvm_x86_ops->hwapic_irr_update(vcpu,
> > > +                           kvm_lapic_find_highest_irr(vcpu));
> > >
> > > Yes, i think so.
> >
> > After adding the "checking ON and setting KVM_REQ_EVENT" operations
> listed in my
> > comments above, do you mean we still need to keep the costly code above
> > inside "if (kvm_check_request(KVM_REQ_EVENT, vcpu) || req_int_win) {}" in
> function
> > vcpu_enter_guest() as it used to be? If yes, my question is what is the exact
> purpose
> > of "checking ON and setting KVM_REQ_EVENT" operations? Here is the code
> flow in
> > vcpu_enter_guest():
> >
> > 1. Check KVM_REQ_EVENT, if it is set, sync pir->virr
> > 2. Disable interrupts
> > 3. Check ON and set KVM_REQ_EVENT -- Here, we set KVM_REQ_EVENT, but
> it is
> > checked in the step 1, which means, we cannot get any benefits even we set it
> here,
> > since the "pir->virr" sync operation was done in step 1, between step 3 and
> VM-Entry,
> > we don't synchronize the pir to virr. So even we set KVM_REQ_EVENT here,
> the interrupts
> > remaining in PIR cannot be delivered to guest during this VM-Entry, right?
> 
> Please check point *1 above. The code will go back to
> 
> "if (kvm_check_request(KVM_REQ_EVENT, vcpu)"
> 
> And perform the pir->virr sync.

Ah, yes, that is the point I was missing. Thanks for pointing this out!

Thanks,
Feng
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