From 40e54cad454076172cc3e2bca60aa924650a3e4b Mon Sep 17 00:00:00 2001 From: Oliver Upton Date: Thu, 25 May 2023 21:27:22 +0000 Subject: KVM: arm64: Document default vPMU behavior on heterogeneous systems KVM maintains a mask of supported CPUs when a vPMU type is explicitly selected by userspace and is used to reject any attempt to run the vCPU on an unsupported CPU. This is great, but we're still beholden to the default behavior where vCPUs can be scheduled anywhere and guest counters may silently stop working. Avoid confusing the next poor sod to look at this code and document the intended behavior. Signed-off-by: Oliver Upton Signed-off-by: Marc Zyngier Link: https://lore.kernel.org/r/20230525212723.3361524-3-oliver.upton@linux.dev --- arch/arm64/kvm/pmu-emul.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) (limited to 'arch') diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c index 5deddc49e745..491ca7eb2a4c 100644 --- a/arch/arm64/kvm/pmu-emul.c +++ b/arch/arm64/kvm/pmu-emul.c @@ -890,7 +890,17 @@ int kvm_arm_pmu_v3_set_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr) return -EBUSY; if (!kvm->arch.arm_pmu) { - /* No PMU set, get the default one */ + /* + * No PMU set, get the default one. + * + * The observant among you will notice that the supported_cpus + * mask does not get updated for the default PMU even though it + * is quite possible the selected instance supports only a + * subset of cores in the system. This is intentional, and + * upholds the preexisting behavior on heterogeneous systems + * where vCPUs can be scheduled on any core but the guest + * counters could stop working. + */ kvm->arch.arm_pmu = kvm_pmu_probe_armpmu(); if (!kvm->arch.arm_pmu) return -ENODEV; -- cgit v1.2.3-70-g09d2