Marc Zyngier <maz@xxxxxxxxxx> writes: > Hi Vitaly, > > On 2021-11-11 16:27, Vitaly Kuznetsov wrote: >> It doesn't make sense to return the recommended maximum number of >> vCPUs which exceeds the maximum possible number of vCPUs. >> >> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> >> --- >> arch/arm64/kvm/arm.c | 7 ++++++- >> 1 file changed, 6 insertions(+), 1 deletion(-) >> >> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c >> index 7838e9fb693e..391dc7a921d5 100644 >> --- a/arch/arm64/kvm/arm.c >> +++ b/arch/arm64/kvm/arm.c >> @@ -223,7 +223,12 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, >> long ext) >> r = 1; >> break; >> case KVM_CAP_NR_VCPUS: >> - r = num_online_cpus(); >> + if (kvm) >> + r = min_t(unsigned int, num_online_cpus(), >> + kvm->arch.max_vcpus); >> + else >> + r = min_t(unsigned int, num_online_cpus(), >> + kvm_arm_default_max_vcpus()); >> break; >> case KVM_CAP_MAX_VCPUS: >> case KVM_CAP_MAX_VCPU_ID: > > This looks odd. This means that depending on the phase userspace is > in while initialising the VM, KVM_CAP_NR_VCPUS can return one thing > or the other. > > For example, I create a VM on a 32 CPU system, NR_VCPUS says 32. > I create a GICv2 interrupt controller, it now says 8. > > That's a change in behaviour that is visible by userspace Yes, I realize this is a userspace visible change. The reason I suggest it is that logically, it seems very odd that the maximum recommended number of vCPUs (KVM_CAP_NR_VCPUS) can be higher, than the maximum supported number of vCPUs (KVM_CAP_MAX_VCPUS). All userspaces which use this information somehow should already contain some workaround for this case. (maybe it's a rare one and nobody hit it yet or maybe there are no userspaces using KVM_CAP_NR_VCPUS for anything besides complaining -- like QEMU). I'd like KVM to be consistent across architectures and have the same (similar) meaning for KVM_CAP_NR_VCPUS. > which I'm keen on avoiding. I'd rather have the kvm and !kvm cases > return the same thing. Forgive me my (ARM?) ignorance but what would it be then? If we go for min(num_online_cpus(), kvm_arm_default_max_vcpus()) in both cases, cat this can still go above KVM_CAP_MAX_VCPUS after vGIC is created? Thanks for the feedback! -- Vitaly
