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

[Marcelo Tosatti]

On Mon, Mar 16, 2015 at 11:42:06AM +0000, Wu, Feng wrote:
> > Do you have any reason why having the code at vcpu_put/vcpu_load is
> > better than the proposal to have the code at kvm_vcpu_block?
> 
> I think your proposal is good, I just want to better understand your idea here.:)

Reduce the overhead of vcpu sched in / vcpu sched out, basically.

> One thing, even we put the code to kvm_vcpu_block, we still need to add code
> at vcpu_put/vcpu_load for the preemption case like what I did now.
> 
> > 
> > >
> > > Global vector is a limited resources in the system, and this involves
> > > common x86 interrupt code changes. I am not sure we can allocate
> > > so many dedicated global vector for KVM usage.
> > 
> > Why not? Have KVM use all free vectors (so if vectors are necessary for
> > other purposes, people should shrink the KVM vector pool).
> 
> If we want to allocate more global vector for this usage, we need hpa's
> input about it. Peter, what is your opinion?

Peter?

> > BTW the Intel docs talk about that ("one vector per vCPU").
> Yes, the Spec talks about this, but it is more complex using one vector per vCPU.
> 
> > 
> > > > > > It seems there is a bunch free:
> > > > > >
> > > > > > commit 52aec3308db85f4e9f5c8b9f5dc4fbd0138c6fa4
> > > > > > Author: Alex Shi <alex.shi@xxxxxxxxx>
> > > > > > Date:   Thu Jun 28 09:02:23 2012 +0800
> > > > > >
> > > > > >     x86/tlb: replace INVALIDATE_TLB_VECTOR by
> > > > CALL_FUNCTION_VECTOR
> > > > > >
> > > > > > Can you add only vcpus which have posted IRTEs that point to this pCPU
> > > > > > to the HLT'ed vcpu lists? (so for example, vcpus without assigned
> > > > > > devices are not part of the list).
> > > > >
> > > > > Is it easy to find whether a vCPU (or the associated domain) has assigned
> > > > devices?
> > > > > If so, we can only add those vCPUs with assigned devices.
> > > >
> > > > When configuring IRTE, at kvm_arch_vfio_update_pi_irte?
> > >
> > > Yes.
> > >
> > > >
> > > > > > > +
> > > > > > >  static int __init vmx_init(void)
> > > > > > >  {
> > > > > > >  	int r, i, msr;
> > > > > > > @@ -9429,6 +9523,8 @@ static int __init vmx_init(void)
> > > > > > >
> > > > > > >  	update_ple_window_actual_max();
> > > > > > >
> > > > > > > +	wakeup_handler_callback = wakeup_handler;
> > > > > > > +
> > > > > > >  	return 0;
> > > > > > >
> > > > > > >  out7:
> > > > > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > > > > > index 0033df3..1551a46 100644
> > > > > > > --- a/arch/x86/kvm/x86.c
> > > > > > > +++ b/arch/x86/kvm/x86.c
> > > > > > > @@ -6152,6 +6152,21 @@ static int vcpu_enter_guest(struct
> > kvm_vcpu
> > > > > > *vcpu)
> > > > > > >  			kvm_vcpu_reload_apic_access_page(vcpu);
> > > > > > >  	}
> > > > > > >
> > > > > > > +	/*
> > > > > > > +	 * Since posted-interrupts can be set by VT-d HW now, in this
> > > > > > > +	 * case, KVM_REQ_EVENT is not set. We move the following
> > > > > > > +	 * operations out of the if statement.
> > > > > > > +	 */
> > > > > > > +	if (kvm_lapic_enabled(vcpu)) {
> > > > > > > +		/*
> > > > > > > +		 * Update architecture specific hints for APIC
> > > > > > > +		 * virtual interrupt delivery.
> > > > > > > +		 */
> > > > > > > +		if (kvm_x86_ops->hwapic_irr_update)
> > > > > > > +			kvm_x86_ops->hwapic_irr_update(vcpu,
> > > > > > > +				kvm_lapic_find_highest_irr(vcpu));
> > > > > > > +	}
> > > > > > > +
> > > > > >
> > > > > > This is a hot fast path. You can set KVM_REQ_EVENT from
> > wakeup_handler.
> > > > >
> > > > > I am afraid Setting KVM_REQ_EVENT from wakeup_handler doesn't help
> > > > much,
> > > > > if vCPU is running in ROOT mode, and VT-d hardware issues an notification
> > > > event,
> > > > > POSTED_INTR_VECTOR interrupt handler will be called.
> > > >
> > > > If vCPU is in root mode, remapping HW will find IRTE configured with
> > > > vector == POSTED_INTR_WAKEUP_VECTOR, use that vector, which will
> > > > VM-exit, and execute the interrupt handler wakeup_handler. Right?
> > >
> > > There are two cases:
> > > Case 1: vCPU is blocked, so it is in root mode, this is what you described
> > above.
> > > Case 2, vCPU is running in root mode, such as, handling vm-exits, in this case,
> > > the notification vector is 'POSTED_INTR_VECTOR', and if external interrupts
> > > from assigned devices happen, the handled of 'POSTED_INTR_VECTOR' will
> > > be called ( it is 'smp_kvm_posted_intr_ipi' in fact), this routine doesn't need
> > > do real things, since the pending interrupts in PIR will be synced to vIRR
> > before
> > > VM-Entry (this code have already been there when enabling CPU-side
> > > posted-interrupt along with APICv). Like what I said before, it is a little hard
> > to
> > > get vCPU related information in it, even if we get, it is not accurate and may
> > harm
> > > the performance.(need search)
> > >
> > > So only setting KVM_REQ_EVENT in wakeup_handler cannot cover the
> > notification
> > > event for 'POSTED_INTR_VECTOR'.
> > >
> > > >
> > > > The point of this comment is that you can keep the
> > > >
> > > > "if (kvm_x86_ops->hwapic_irr_update)
> > > > 	kvm_x86_ops->hwapic_irr_update(vcpu,
> > > > 			kvm_lapic_find_highest_irr(vcpu));
> > > > "
> > > >
> > > > Code inside KVM_REQ_EVENT handling section of vcpu_run, as long as
> > > > wakeup_handler sets KVM_REQ_EVENT.
> > >
> > > Please see above.
> > 
> > OK can you set KVM_REQ_EVENT in case the ON bit is set,
> > after disabling interrupts ?
> > 
> 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.

> 
> "if (kvm_x86_ops->hwapic_irr_update)
> 	kvm_x86_ops->hwapic_irr_update(vcpu,
> 			kvm_lapic_find_highest_irr(vcpu));
> 
> > kvm_lapic_find_highest_irr(vcpu) eats some cache
> > (4 cachelines) versus 1 cacheline for reading ON bit.
> > 
> > > > > > Please remove blocked and wakeup_cpu, they should not be necessary.
> > > > >
> > > > > Why do you think wakeup_cpu is not needed, when vCPU is blocked,
> > > > > wakeup_cpu saves the cpu which the vCPU is blocked on, after vCPU
> > > > > is woken up, it can run on a different cpu, so we need wakeup_cpu to
> > > > > find the right list to wake up the vCPU.
> > > >
> > > > If the vCPU was moved it should have updated IRTE destination field
> > > > to the pCPU which it has moved to?
> > >
> > > Every time a vCPU is scheduled to a new pCPU, the IRTE destination filed
> > > would be updated accordingly.
> > >
> > > When vCPU is blocked. To wake up the blocked vCPU, we need to find which
> > > list the vCPU is blocked on, and this is what wakeup_cpu used for?
> > 
> > Right, perhaps prev_vcpu is a better name.
> 
> Do you mean "prev_pcpu"?

Yes.


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