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

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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