RE: [RFC PATCH v4 5/7] KVM: x86: add vCPU scoped toggling for disabled exits

[Kechen Lu]

> -----Original Message-----
> From: Sean Christopherson <seanjc@xxxxxxxxxx>
> Sent: Wednesday, July 20, 2022 12:30 PM
> To: Kechen Lu <kechenl@xxxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx; pbonzini@xxxxxxxxxx; chao.gao@xxxxxxxxx;
> vkuznets@xxxxxxxxxx; Somdutta Roy <somduttar@xxxxxxxxxx>; linux-
> kernel@xxxxxxxxxxxxxxx
> Subject: Re: [RFC PATCH v4 5/7] KVM: x86: add vCPU scoped toggling for
> disabled exits
> 
> External email: Use caution opening links or attachments
> 
> 
> On Wed, Jul 20, 2022, Kechen Lu wrote:
> > > > @@ -6036,14 +6045,17 @@ int kvm_vm_ioctl_enable_cap(struct kvm
> kvm,
> > > >                       break;
> > > >
> > > >               mutex_lock(&kvm->lock);
> > > > -             if (kvm->created_vcpus)
> > > > -                     goto disable_exits_unlock;
> > > > +             if (kvm->created_vcpus) {
> > >
> > > I retract my comment about using a request, I got ahead of myself.
> > >
> > > Don't update vCPUs, the whole point of adding the
> > > !kvm->created_vcpus check was to avoid having to update vCPUs when
> > > the per-VM behavior changed.
> > >
> > > In other words, keep the restriction and drop the request.
> > >
> >
> > I see. If we keep the restriction here and not updating vCPUs when
> > kvm->created_vcpus is true, the per-VM and per-vCPU assumption would
> > kvm->be
> > different here? Not sure if I understand right:
> > For per-VM, we assume the per-VM cap enabling is only before vcpus
> creation.
> > For per-vCPU cap enabling, we are able to toggle the disabled exits runtime.
> 
> Yep.  The main reason being that there's no use case for changing per-VM
> settings after vCPUs are created.  I.e. we could lift the restriction in the future
> if a use case pops up, but until then, keep things simple.
> 
> > If I understand correctly, this also makes sense though.
> 
> Paging this all back in...
> 
> There are two (sane) options for defining KVM's ABI:
> 
>   1) KVM combines the per-VM and per-vCPU settings
>   2) The per-vCPU settings override the per-VM settings
> 
> This series implements (2).
> 
> For (1), KVM would need to recheck the per-VM state during the per-vCPU
> update, e.g. instead of simply modifying the per-vCPU flags, the vCPU-scoped
> handler for KVM_CAP_X86_DISABLE_EXITS would need to merge the
> incoming settings with the existing kvm->arch.xxx_in_guest flags.
> 
> I like (2) because it's simpler to implement and document (merging state is
> always
> messy) and is more flexible.  E.g. with (1), the only way to have per-vCPU
> settings is for userspace to NOT set the per-VM disables and then set
> disables on a per-vCPU basis.  Whereas with (2), userspace can set (or not)
> the per-VM disables and then override as needed.

Gotcha. Makes sense to me. Thanks for the elaboration!

BR,
Kechen