Re: [PATCH v3 51/59] KVM: arm/arm64: GICv4: Add doorbell interrupt handling

[Christoffer Dall]

On Thu, Aug 31, 2017 at 01:18:20PM +0100, Marc Zyngier wrote:
> On 28/08/17 19:18, Christoffer Dall wrote:
> > On Fri, Aug 04, 2017 at 08:44:04AM +0100, Marc Zyngier wrote:
> >> On 31/07/17 18:26, Marc Zyngier wrote:
> >>> When a vPE is not running, a VLPI being made pending results in a
> >>> doorbell interrupt being delivered. Let's handle this interrupt
> >>> and update the pending_last flag that indicates that VLPIs are
> >>> pending. The corresponding vcpu is also kicked into action.
> >>>
> >>> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
> >>> ---
> >>>  virt/kvm/arm/vgic/vgic-v4.c | 34 ++++++++++++++++++++++++++++++++++
> >>>  1 file changed, 34 insertions(+)
> >>>
> >>> diff --git a/virt/kvm/arm/vgic/vgic-v4.c b/virt/kvm/arm/vgic/vgic-v4.c
> >>> index 534d3051a078..6af3cde6d7d4 100644
> >>> --- a/virt/kvm/arm/vgic/vgic-v4.c
> >>> +++ b/virt/kvm/arm/vgic/vgic-v4.c
> >>> @@ -21,6 +21,19 @@
> >>>  
> >>>  #include "vgic.h"
> >>>  
> >>> +static irqreturn_t vgic_v4_doorbell_handler(int irq, void *info)
> >>> +{
> >>> +	struct kvm_vcpu *vcpu = info;
> >>> +
> >>> +	if (!kvm_vgic_vcpu_pending_irq(vcpu)) {
> >>> +		vcpu->arch.vgic_cpu.vgic_v3.its_vpe.pending_last = true;
> >>> +		kvm_make_request(KVM_REQ_IRQ_PENDING, vcpu);
> >>> +		kvm_vcpu_kick(vcpu);
> >>> +	}
> >>
> >> This code is so obviously broken that I completely overlooked it.
> >>
> >> If we have take a doorbell interrupt, then it means nothing was
> >> otherwise pending (because we'd have been kicked out of the blocking
> >> state, and will have masked the doorbell). So checking for pending
> >> interrupts is pointless.
> >>
> >> Furthermore, calling kvm_vgic_vcpu_pending_irq() takes the ap_list
> >> lock. If we take a doorbell interrupt while injecting a virtual
> >> interrupt (from userspace, for example) on the same CPU, we end-up
> >> in deadlock land. This would be solved by Christoffer's latest
> >> crop of timer patches, but there is no point getting there the first
> >> place.
> >>
> >> The patchlet below solves it:
> >>
> >> diff --git a/virt/kvm/arm/vgic/vgic-v4.c b/virt/kvm/arm/vgic/vgic-v4.c
> >> index 15feb1151797..48e4d6ebeaa8 100644
> >> --- a/virt/kvm/arm/vgic/vgic-v4.c
> >> +++ b/virt/kvm/arm/vgic/vgic-v4.c
> >> @@ -94,11 +94,9 @@ static irqreturn_t vgic_v4_doorbell_handler(int irq, void *info)
> >>  {
> >>  	struct kvm_vcpu *vcpu = info;
> >>  
> >> -	if (!kvm_vgic_vcpu_pending_irq(vcpu)) {
> >> -		vcpu->arch.vgic_cpu.vgic_v3.its_vpe.pending_last = true;
> >> -		kvm_make_request(KVM_REQ_IRQ_PENDING, vcpu);
> >> -		kvm_vcpu_kick(vcpu);
> >> -	}
> >> +	vcpu->arch.vgic_cpu.vgic_v3.its_vpe.pending_last = true;
> >> +	kvm_make_request(KVM_REQ_IRQ_PENDING, vcpu);
> >> +	kvm_vcpu_kick(vcpu);
> > 
> > I don't think you need the request and kick, because if you're getting
> > this doorbell, doesn't that also mean that the VCPU is not running in
> > the guest and you simply need to make sure the VCPU thread gets
> > scheduled again, so you could call kvm_vcpu_wake_up() instead.
> > 
> > Unless the request is there to ensure proper memory barriers around
> > setting pending_last?
> 
> We definitely need some form of barrier here (the write needs to be
> observed on the CPU we're waking up on), and I wanted to model this in a
> similar way to our injection path (where we do have the make_request +
> kick pattern).
> 
> After all, we really have an interrupt pending that deserves attention.
> Is there any reason why we'd want to model it differently from the rest
> of the injection path?
> 

The difference is that you'll never need this if the VCPU is running,
this is only when the VCPU is on a waitqueue and sleeping, and therefore
you don't actually need a kick, but you just need to wake up the VCPU
thread.

But kvm_vcpu_kick() also wakes up, it's just a question of using the
most appropriate hammer, making it clear what exactly is going on.

But, as Drew pointed out, we use the request framework to ensure proper
barriers (when he fixes it), after waking up a VCPU, so in this cae it's
to make sure the VCPU thread observed pending_last==true after being
woken up.

Interestingly, we don't need that for the timer, since we don't change
random in-memory values, but we read the physical counter which is the
thing that changes when waking up the thread.

So, sorry for a lot of noise, your code is fine, and Drew fixes KVM, I
think.

Thanks,
-Christoffer