Re: [PATCH v3 3/4] x86/kvm: add max number of vcpus for hyperv emulation

[Sean Christopherson <seanjc@google.com>]

On Thu, Nov 18, 2021, Juergen Gross wrote:
> On 17.11.21 21:50, Sean Christopherson wrote:
> > > @@ -166,7 +166,7 @@ static struct kvm_vcpu *get_vcpu_by_vpidx(struct kvm *kvm, u32 vpidx)
> > >   	struct kvm_vcpu *vcpu = NULL;
> > >   	int i;
> > > -	if (vpidx >= KVM_MAX_VCPUS)
> > > +	if (vpidx >= min(KVM_MAX_VCPUS, KVM_MAX_HYPERV_VCPUS))
> > 
> > IMO, this is conceptually wrong.  KVM should refuse to allow Hyper-V to be enabled
> > if the max number of vCPUs exceeds what can be supported, or should refuse to create
> 
> TBH, I wasn't sure where to put this test. Is there a guaranteed
> sequence of ioctl()s regarding vcpu creation (or setting the max
> number of vcpus) and the Hyper-V enabling?

For better or worse (mostly worse), like all other things CPUID, Hyper-V is a per-vCPU
knob.  If KVM can't detect the impossible condition at compile time, kvm_check_cpuid()
is probably the right place to prevent enabling Hyper-V on an unreachable vCPU.

> > the vCPUs.  I agree it makes sense to add a Hyper-V specific limit, since there are
> > Hyper-V structures that have a hard limit, but detection of violations should be a
> > BUILD_BUG_ON, not a silent failure at runtime.
> > 
> 
> A BUILD_BUG_ON won't be possible with KVM_MAX_VCPUS being selecteble via
> boot parameter.

I was thinking that there would still be a KVM-defined max that would cap whatever
comes in from userspace.