Subject: [PATCH 1/5] KVM: arm64: Allow creating the PMU without the in-kernel GIC

From: [email-masked] (Christoffer Dall)
Date: Thu, 4 May 2017 10:38:45 +0200

On Thu, May 04, 2017 at 09:28:50AM +0100, Marc Zyngier wrote:
> On 04/05/17 09:13, Christoffer Dall wrote:
> > On Thu, May 04, 2017 at 09:09:47AM +0100, Marc Zyngier wrote:
> >> On 03/05/17 19:32, 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                         | 27 +++++++++++++++++----------
> >>>  2 files changed, 26 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
> >>>           -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..f046b08 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;
> >>> +	}
> >>> +
> >>>  	kvm_pmu_vcpu_reset(vcpu);
> >>>  	vcpu->arch.pmu.ready = true;
> >>>  
> >>> @@ -512,6 +516,9 @@ int kvm_arm_pmu_v3_set_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr)
> >>>  		int __user *uaddr = (int __user *)(long)attr->addr;
> >>>  		int irq;
> >>>  
> >>> +		if (!irqchip_in_kernel(vcpu->kvm))
> >>> +			return -EINVAL;
> >>> +
> >>
> >> Shouldn't we fail the same way for {get,has}_attr? get_attr is going to
> >> generate a -ENXIO, and has_attr is going to lie about the availability
> >> of KVM_ARM_VCPU_PMU_V3_IRQ...
> >>
> > 
> > Here's the text from api.txt:
> > 
> >   Tests whether a device supports a particular attribute.  A successful
> >   return indicates the attribute is implemented.  It does not necessarily
> >   indicate that the attribute can be read or written in the device's
> >   current state.  "addr" is ignored.
> > 
> > My interpretation therefore is that QEMU can use this ioctl to figure
> > out if the feature is supported (sort of like a capability), but that
> > doesn't mean that the configuration of the VM is such that the attribute
> > can be get or set at that moment.
> > 
> > For example, there will also alway be situations where you can get an
> > attr, but not set an attr, what should the has_attr return then?
> 
> My issue here is that whether we can get/set the interrupt or not is not
> a function of the device itself, but of the way it is "wired". No matter
> what "the device's current state" is, we'll never be able to get/set the
> interrupt.
> 
> I'd tend to err on the side of caution and return something that is
> unambiguous, be maybe I have too strict an interpretation of the API.
> 

Hmm, I see the has_attr as a method for userspace to discover "does this
kernel have this feature".  If we make has_attr return a value depending
on the VM having an in-kernel gic or not, we implicitly require
userspace to create a VM with an in-kernel GIC to discover if this
kernel has that feature, and therefore also impose an ordering
requirement of figuring out the capabilities of the kernel (i.e. create
the GIC before checking this API).

I think QEMU should be able to do:

  if (has_attr()) {
     kvm_supports_set_timer_irq = true;
     vtimer_irq = foo;
  } else {
     kvm_supports_set_timer_irq = false;
     vtimer_irq = 27; /* default, we're stuck with it */
  }

  create_board_definition();
  create_dt();
  create_acpi();

  /* do whatever */

  if (kvm_supports_set_timer_irq && kvm_irqchip_in_kernel()) {
  	kvm_arm_timer_set_irq(...);
  }

And all this should not be coupled to when we create the irqchip device.

But I may be failing to see the case where the current implementation
creates a problem for userspace, in which case we should document the
ordering requirement.

Note: that I don't think it's expected that has_attr implies get_attr
and set_attr succeed.  For example, has_attr is currently typically
called with a null pointer to the addr field.

Thanks,
-Christoffer