Re: [PATCH v3 3/4] x86/kvm: add max number of vcpus for hyperv emulation
From: Sean Christopherson
Date: Thu Nov 18 2021 - 09:49:42 EST
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.