Re: [PATCH 1/5] KVM: arm64: Cap KVM_CAP_NR_VCPUS by KVM_CAP_MAX_VCPUS

[Andrew Jones <drjones@redhat.com>]

On Fri, Nov 12, 2021 at 10:51:10AM +0100, Vitaly Kuznetsov wrote:
> Marc Zyngier <maz@kernel.org> 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@redhat.com>
> >> ---
> >>  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.

KVM_CAP_NR_VCPUS seems pretty useless if we just want to tell userspace
the same thing it can get with _SC_NPROCESSORS_ONLN. In fact, if userspace
knows something we don't about the future onlining of some pcpus, then
maybe userspace would prefer to check _SC_NPROCESSORS_CONF.

> 
> > 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?

So the GIC version case looks like the type of thing that could make
KVM_CAP_NR_VCPUS useful, i.e. being able to tell userspace a maximum
number of vcpus supported for a given configuration. However, even
in that case the concept of "recommended" number doesn't make sense,
because, for the GICv2 example, a VM cannot configure more than 8 VCPUs,
so it's a real limit, not a recommendation. Maybe KVM_CAP_NR_VCPUS should
just be left alone, but deprecated, and, if there's need, a new CAP could
be created for a config-vcpu-max.

Thanks,
drew