Re: [RFC PATCH v1 3/4] KVM: x86: nSVM: Implement support for nested Bus Lock Threshold

[Sean Christopherson <seanjc@google.com>]

On Tue, Jul 09, 2024, Manali Shukla wrote:
> @@ -758,6 +759,16 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
>  		}
>  	}
>  
> +	/*
> +	 * If guest intercepts BUSLOCK, use guest's bus_lock_counter value,
> +	 * otherwise use host bus_lock_counter value.
> +	 */
> +	if (guest_can_use(vcpu, X86_FEATURE_BUS_LOCK_THRESHOLD) &&
> +	    vmcb12_is_intercept(&svm->nested.ctl, INTERCEPT_BUSLOCK))
> +		vmcb02->control.bus_lock_counter = svm->nested.ctl.bus_lock_counter;
> +	else
> +		vmcb02->control.bus_lock_counter = vmcb01->control.bus_lock_counter;

Copying vmcb01's count to/from vmcb02 belongs in the core enabling patch.  From
KVM's perspective, the counter is associated with a vCPU, not a VMCB, and so the
count should keep running across nested transitions.

As written, taking only the core enabling patch will mean that L2 runs with the
wrong count.  Amusingly, because '0' means "always exit", L2 would run in a *more*
restrictive environment due to the VMCB being zero-allocated.