KVM: SVM: Emulate #INIT in response to triple fault shutdown

Emulate a full #INIT instead of simply initializing the VMCB if the
guest hits a shutdown.  Initializing the VMCB but not other vCPU state,
much of which is mirrored by the VMCB, results in incoherent and broken
vCPU state.

Ideally, KVM would not automatically init anything on shutdown, and
instead put the vCPU into e.g. KVM_MP_STATE_UNINITIALIZED and force
userspace to explicitly INIT or RESET the vCPU.  Even better would be to
add KVM_MP_STATE_SHUTDOWN, since technically NMI can break shutdown
(and SMI on Intel CPUs).

But, that ship has sailed, and emulating #INIT is the next best thing as
that has at least some connection with reality since there exist bare
metal platforms that automatically INIT the CPU if it hits shutdown.

Fixes: 46fe4ddd ("[PATCH] KVM: SVM: Propagate cpu shutdown events to userspace")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210713163324.627647-45-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/svm.c

@@ -2050,11 +2050,15 @@ static int shutdown_interception(struct kvm_vcpu *vcpu)
 		return -EINVAL;
 
 	/*
-	 * VMCB is undefined after a SHUTDOWN intercept
-	 * so reinitialize it.
+	 * VMCB is undefined after a SHUTDOWN intercept.  INIT the vCPU to put
+	 * the VMCB in a known good state.  Unfortuately, KVM doesn't have
+	 * KVM_MP_STATE_SHUTDOWN and can't add it without potentially breaking
+	 * userspace.  At a platform view, INIT is acceptable behavior as
+	 * there exist bare metal platforms that automatically INIT the CPU
+	 * in response to shutdown.
 	 */
 	clear_page(svm->vmcb);
-	init_vmcb(vcpu);
+	kvm_vcpu_reset(vcpu, true);
 
 	kvm_run->exit_reason = KVM_EXIT_SHUTDOWN;
 	return 0;

arch/x86/kvm/x86.c

@@ -10906,6 +10906,7 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
 	if (init_event)
 		kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu);
 }
+EXPORT_SYMBOL_GPL(kvm_vcpu_reset);
 
 void kvm_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector)
 {