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.
