Subject: [PATCH v2 1/9] KVM: arm64: Allow creating the PMU without the in-kernel GIC

From: [email-masked] (Christoffer Dall)
Date: Wed, 24 May 2017 10:38:05 +0200

On Tue, May 23, 2017 at 05:52:01PM +0100, Marc Zyngier wrote:
> On 16/05/17 19:45, Christoffer Dall wrote:
> > Since we got support for devices in userspace which allows reporting the
> > PMU overflow output status to userspace, we should actually allow
> > creating the PMU on systems without an in-kernel irqchip, which in turn
> > requires us to slightly clarify error codes for the ABI and move things
> > around for the initialization phase.
> > 
> > Signed-off-by: Christoffer Dall <[email-masked]>
> > ---
> >  Documentation/virtual/kvm/devices/vcpu.txt | 16 +++++++++-------
> >  virt/kvm/arm/pmu.c                         | 30 ++++++++++++++++++++----------
> >  2 files changed, 29 insertions(+), 17 deletions(-)
> > 
> > diff --git a/Documentation/virtual/kvm/devices/vcpu.txt b/Documentation/virtual/kvm/devices/vcpu.txt
> > index 02f5068..352af6e 100644
> > --- a/Documentation/virtual/kvm/devices/vcpu.txt
> > +++ b/Documentation/virtual/kvm/devices/vcpu.txt
> > @@ -16,7 +16,9 @@ Parameters: in kvm_device_attr.addr the address for PMU overflow interrupt is a
> >  Returns: -EBUSY: The PMU overflow interrupt is already set
> >           -ENXIO: The overflow interrupt not set when attempting to get it
> >           -ENODEV: PMUv3 not supported
> > -         -EINVAL: Invalid PMU overflow interrupt number supplied
> > +         -EINVAL: Invalid PMU overflow interrupt number supplied or
> > +                  trying to set the IRQ number without using an in-kernel
> > +                  irqchip.
> >  
> >  A value describing the PMUv3 (Performance Monitor Unit v3) overflow interrupt
> >  number for this vcpu. This interrupt could be a PPI or SPI, but the interrupt
> > @@ -25,11 +27,11 @@ all vcpus, while as an SPI it must be a separate number per vcpu.
> >  
> >  1.2 ATTRIBUTE: KVM_ARM_VCPU_PMU_V3_INIT
> >  Parameters: no additional parameter in kvm_device_attr.addr
> > -Returns: -ENODEV: PMUv3 not supported
> > -         -ENXIO: PMUv3 not properly configured as required prior to calling this
> > -                 attribute
> > +Returns: -ENODEV: PMUv3 not supported or GIC not initialized
> > +         -ENXIO: PMUv3 not properly configured or in-kernel irqchip not
> > +                 conigured as required prior to calling this attribute
> 
>                     configured
> 
> >           -EBUSY: PMUv3 already initialized
> >  
> > -Request the initialization of the PMUv3.  This must be done after creating the
> > -in-kernel irqchip.  Creating a PMU with a userspace irqchip is currently not
> > -supported.
> > +Request the initialization of the PMUv3.  If using the PMUv3 with an in-kernel
> > +virtual GIC implementation, this must be done after initializing the in-kernel
> > +irqchip.
> > diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
> > index 4b43e7f..7209185 100644
> > --- a/virt/kvm/arm/pmu.c
> > +++ b/virt/kvm/arm/pmu.c
> > @@ -456,21 +456,25 @@ static int kvm_arm_pmu_v3_init(struct kvm_vcpu *vcpu)
> >  	if (!kvm_arm_support_pmu_v3())
> >  		return -ENODEV;
> >  
> > -	/*
> > -	 * We currently require an in-kernel VGIC to use the PMU emulation,
> > -	 * because we do not support forwarding PMU overflow interrupts to
> > -	 * userspace yet.
> > -	 */
> > -	if (!irqchip_in_kernel(vcpu->kvm) || !vgic_initialized(vcpu->kvm))
> > -		return -ENODEV;
> > -
> > -	if (!test_bit(KVM_ARM_VCPU_PMU_V3, vcpu->arch.features) ||
> > -	    !kvm_arm_pmu_irq_initialized(vcpu))
> > +	if (!test_bit(KVM_ARM_VCPU_PMU_V3, vcpu->arch.features))
> >  		return -ENXIO;
> >  
> >  	if (kvm_arm_pmu_v3_ready(vcpu))
> >  		return -EBUSY;
> >  
> > +	if (irqchip_in_kernel(vcpu->kvm)) {
> > +		/*
> > +		 * If using the PMU with an in-kernel virtual GIC
> > +		 * implementation, we require the GIC to be already
> > +		 * initialized when initializing the PMU.
> > +		 */
> > +		if (!vgic_initialized(vcpu->kvm))
> > +			return -ENODEV;
> > +
> > +		if (!kvm_arm_pmu_irq_initialized(vcpu))
> > +			return -ENXIO;
> > +	}
> > +
> 
> Do we also need to prevent a vgic to be created if the PMU has been
> initialized beforehand?
> 

Sigh.  We probably have to.

I don't like having a cross-VGIC-PMU check, but we could do something
like setting a flag on the kvm struct so that irqchip_in_user() always
return true, and if that is set, it is not possible to create the VGIC.

Alternatively we can make the PMU init a no-op, and try to enable it via
the first-vcpu-run path, like the timer, and check that everything lines
up then (i.e. you have in-kernel irqchip with a non-conflicting irq
number or you have a userspace irqchip).

Thougths?

Thanks,
-Christoffer