KVM: arm64: gic-v5: Mandate architected PPI for PMU emulation on GICv5

Make it mandatory to use the architected PPI when running a GICv5
guest. Attempts to set anything other than the architected PPI (23)
are rejected.

Additionally, KVM_ARM_VCPU_PMU_V3_INIT is relaxed to no longer require
KVM_ARM_VCPU_PMU_V3_IRQ to be called for GICv5-based guests. In this
case, the architectued PPI is automatically used.

Documentation is bumped accordingly.

Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Link: https://patch.msgid.link/20260319154937.3619520-33-sascha.bischoff@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

Documentation/virt/kvm/devices/vcpu.rst

@@ -37,7 +37,8 @@ Returns:
 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
 type must be same for each vcpu. As a PPI, the interrupt number is the same for
-all vcpus, while as an SPI it must be a separate number per vcpu.
+all vcpus, while as an SPI it must be a separate number per vcpu. For
+GICv5-based guests, the architected PPI (23) must be used.
 
 1.2 ATTRIBUTE: KVM_ARM_VCPU_PMU_V3_INIT
 ---------------------------------------
@@ -50,7 +51,7 @@ Returns:
 	 -EEXIST  Interrupt number already used
 	 -ENODEV  PMUv3 not supported or GIC not initialized
 	 -ENXIO   PMUv3 not supported, missing VCPU feature or interrupt
-		  number not set
+		  number not set (non-GICv5 guests, only)
 	 -EBUSY   PMUv3 already initialized
 	 =======  ======================================================