Re: [PATCH 2/4] KVM: x86: Rely solely on preempted_in_kernel flag for directed yield

[Yuan Yao]

On Wed, Jan 10, 2024 at 09:13:28AM -0800, Sean Christopherson wrote:
> On Wed, Jan 10, 2024, Yuan Yao wrote:
> > On Tue, Jan 09, 2024 at 04:39:36PM -0800, Sean Christopherson wrote:
> > > @@ -13093,7 +13092,7 @@ bool kvm_arch_dy_has_pending_interrupt(struct kvm_vcpu *vcpu)
> > >
> > >  bool kvm_arch_vcpu_preempted_in_kernel(struct kvm_vcpu *vcpu)
> > >  {
> > > -	return kvm_arch_vcpu_in_kernel(vcpu);
> > > +	return vcpu->arch.preempted_in_kernel;
> > >  }
> > >
> > >  bool kvm_arch_dy_runnable(struct kvm_vcpu *vcpu)
> > > @@ -13116,9 +13115,6 @@ bool kvm_arch_vcpu_in_kernel(struct kvm_vcpu *vcpu)
> > >  	if (vcpu->arch.guest_state_protected)
> > >  		return true;
> > >
> > > -	if (vcpu != kvm_get_running_vcpu())
> > > -		return vcpu->arch.preempted_in_kernel;
> > > -
> >
> > Now this function accepts vcpu parameter but can only get information from
> > "current" vcpu loaded on hardware for VMX.  I'm not sure whether need
> > "WARN_ON(vcpu != kvm_get_running_vcpu())" here to guard it. i.e.
> > kvm_guest_state() still uses this function (although it did chekcing before).
>
> Eh, I don't think it's worth adding a one-off kvm_get_running_vcpu() sanity check.
> In the vast majority of cases, if VMREAD or VMWRITE is used improperly, the
> instruction will fail at some point due to the pCPU not having any VMCS loaded.
> It's really just cross-vCPU checks that could silently do the wrong thing, and
> those flows are so few and far between that I'm comfortable taking a "just get
> it right stance".
>
> If we want to add sanity checks, I think my vote would be to plumb @vcpu down
> into vmcs_read{16,32,64,l} and add sanity checks there, probably with some sort
> of guard so that the sanity checks can be enabled only for debug kernels.

I got your point.

Reviewed-by: Yuan Yao <yuan.yao@xxxxxxxxx>