KVM: TDX: Don't offline the last cpu of one package when there's TDX guest

Destroying TDX guest requires there's at least one cpu online for each
package, because reclaiming the TDX KeyID of the guest (as part of the
teardown process) requires to call some SEAMCALL (on any cpu) on all
packages.

Do not offline the last cpu of one package when there's any TDX guest
running, otherwise KVM may not be able to teardown TDX guest resulting
in leaking of TDX KeyID and other resources like TDX guest control
structure pages.

Implement the TDX version 'offline_cpu()' to prevent the cpu from going
offline if it is the last cpu on the package.

Co-developed-by: Kai Huang <[e-mail]>
Signed-off-by: Kai Huang <[e-mail]>
Suggested-by: Sean Christopherson <[e-mail]>
Signed-off-by: Isaku Yamahata <[e-mail]>
Signed-off-by: Rick Edgecombe <[e-mail]>
Reviewed-by: Binbin Wu <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

arch/x86/kvm/vmx/tdx.c

···
  */
 static DEFINE_MUTEX(tdx_lock);
 
+static atomic_t nr_configured_hkid;
+
 static inline void tdx_hkid_free(struct kvm_tdx *kvm_tdx)
 {
 	tdx_guest_keyid_free(kvm_tdx->hkid);
 	kvm_tdx->hkid = -1;
+	atomic_dec(&nr_configured_hkid);
 }
 
 static inline bool is_hkid_assigned(struct kvm_tdx *kvm_tdx)
···
 
 	ret = -ENOMEM;
 
+	atomic_inc(&nr_configured_hkid);
+
 	tdr_page = alloc_page(GFP_KERNEL);
 	if (!tdr_page)
 		goto free_hkid;
···
 	return r;
 }
 
+static int tdx_offline_cpu(unsigned int cpu)
+{
+	int i;
+
+	/* No TD is running.  Allow any cpu to be offline. */
+	if (!atomic_read(&nr_configured_hkid))
+		return 0;
+
+	/*
+	 * In order to reclaim TDX HKID, (i.e. when deleting guest TD), need to
+	 * call TDH.PHYMEM.PAGE.WBINVD on all packages to program all memory
+	 * controller with pconfig.  If we have active TDX HKID, refuse to
+	 * offline the last online cpu.
+	 */
+	for_each_online_cpu(i) {
+		/*
+		 * Found another online cpu on the same package.
+		 * Allow to offline.
+		 */
+		if (i != cpu && topology_physical_package_id(i) ==
+				topology_physical_package_id(cpu))
+			return 0;
+	}
+
+	/*
+	 * This is the last cpu of this package.  Don't offline it.
+	 *
+	 * Because it's hard for human operator to understand the
+	 * reason, warn it.
+	 */
+#define MSG_ALLPKG_ONLINE \
+	"TDX requires all packages to have an online CPU. Delete all TDs in order to offline all CPUs of a package.\n"
+	pr_warn_ratelimited(MSG_ALLPKG_ONLINE);
+	return -EBUSY;
+}
+
 static void __do_tdx_cleanup(void)
 {
 	/*
···
 	 */
 	r = cpuhp_setup_state_cpuslocked(CPUHP_AP_ONLINE_DYN,
 					 "kvm/cpu/tdx:online",
-					 tdx_online_cpu, NULL);
+					 tdx_online_cpu, tdx_offline_cpu);
 	if (r < 0)
 		return r;