KVM: SVM: Add a comment to clarify what vcpu_svm.vmcb points at

Add a comment above the declaration of vcpu_svm.vmcb to call out that it
is simply a shorthand for current_vmcb->ptr.  The myriad accesses to
svm->vmcb are quite confusing without this crucial detail.

No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210406171811.4043363-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/svm.h

@@ -110,6 +110,7 @@ struct svm_nested_state {
 
 struct vcpu_svm {
 	struct kvm_vcpu vcpu;
+	/* vmcb always points at current_vmcb->ptr, it's purely a shorthand. */
 	struct vmcb *vmcb;
 	struct kvm_vmcb_info vmcb01;
 	struct kvm_vmcb_info *current_vmcb;