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


> -----Original Message-----
> From: Marcelo Tosatti [mailto:mtosatti@redhat.com]
> Sent: Tuesday, March 31, 2015 7:56 AM
> To: Wu, Feng
> Cc: hpa@zytor.com; tglx@linutronix.de; mingo@redhat.com; x86@kernel.org;
> gleb@kernel.org; pbonzini@redhat.com; dwmw2@infradead.org;
> joro@8bytes.org; alex.williamson@redhat.com; jiang.liu@linux.intel.com;
> eric.auger@linaro.org; linux-kernel@vger.kernel.org;
> iommu@lists.linux-foundation.org; kvm@vger.kernel.org
> 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@redhat.com]
> > > Sent: Saturday, March 28, 2015 3:30 AM
> > > To: Wu, Feng
> > > Cc: hpa@zytor.com; tglx@linutronix.de; mingo@redhat.com;
> x86@kernel.org;
> > > gleb@kernel.org; pbonzini@redhat.com; dwmw2@infradead.org;
> > > joro@8bytes.org; alex.williamson@redhat.com; jiang.liu@linux.intel.com;
> > > eric.auger@linaro.org; linux-kernel@vger.kernel.org;
> > > iommu@lists.linux-foundation.org; kvm@vger.kernel.org
> > > 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);
>

Hi Marcelo,

I changed the code as above, then I found that the ping latency was extremely big, (70ms - 400ms).
I digged into it and got the root cause. We cannot use "checking-on" as the judgment, since 'ON'
can be cleared by hypervisor software in lots of places. In this case, KVM_REQ_EVENT cannot be
set when we check 'ON' bit, hence the interrupts are not injected to the guest in time.

Please refer to the following code, in which 'ON' bit can be cleared:

apic_find_highest_irr () --> vmx_sync_pir_to_irr () --> pi_test_and_clear_on()

Searching from the code step by step, apic_find_highest_irr() can be called by many other guys.

Thanks,
Feng

> 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.



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