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

[Juergen Gross]

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

Attachment: OpenPGP_0xB0DE9DD628BF132F.asc
Description: OpenPGP public key

Attachment: OpenPGP_signature
Description: OpenPGP digital signature