On 18.11.21 15:49, Sean Christopherson wrote:
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 createTBH, 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.
With HYPERV_CPUID_IMPLEMENT_LIMITS already returning the supported number of vcpus for the Hyper-V case I'm not sure there is really more needed. The problem I'm seeing is that the only thing I can do is to let kvm_get_hv_cpuid() not adding the Hyper-V cpuid leaves for vcpus > 64. I can't return a failure, because that would probably let vcpu creation fail. And this is something we don't want, as kvm_get_hv_cpuid() is called even in the case the guest doesn't plan to use Hyper-V extensions.
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.
See my answers to you your other responses. Juergen
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