KVM: SEV: Mark nested locking of vcpu->lock

svm_vm_migrate_from() uses sev_lock_vcpus_for_migration() to lock all
source and target vcpu->locks. Unfortunately there is an 8 subclass
limit, so a new subclass cannot be used for each vCPU. Instead maintain
ownership of the first vcpu's mutex.dep_map using a role specific
subclass: source vs target. Release the other vcpu's mutex.dep_maps.

Fixes: b56639318bb2b ("KVM: SEV: Add support for SEV intra host migration")
Reported-by: John Sperbeck<jsperbeck@google.com>
Suggested-by: David Rientjes <rientjes@google.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Hillf Danton <hdanton@sina.com>
Cc: kvm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Peter Gonda <pgonda@google.com>

Message-Id: <20220502165807.529624-1-pgonda@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 0ad70c12..7c39287 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -1594,24 +1594,51 @@
 	atomic_set_release(&src_sev->migration_in_progress, 0);
 }
 
+/* vCPU mutex subclasses.  */
+enum sev_migration_role {
+	SEV_MIGRATION_SOURCE = 0,
+	SEV_MIGRATION_TARGET,
+	SEV_NR_MIGRATION_ROLES,
+};
 
-static int sev_lock_vcpus_for_migration(struct kvm *kvm)
+static int sev_lock_vcpus_for_migration(struct kvm *kvm,
+					enum sev_migration_role role)
 {
 	struct kvm_vcpu *vcpu;
 	unsigned long i, j;
+	bool first = true;
 
 	kvm_for_each_vcpu(i, vcpu, kvm) {
-		if (mutex_lock_killable(&vcpu->mutex))
+		if (mutex_lock_killable_nested(&vcpu->mutex, role))
 			goto out_unlock;
+
+		if (first) {
+			/*
+			 * Reset the role to one that avoids colliding with
+			 * the role used for the first vcpu mutex.
+			 */
+			role = SEV_NR_MIGRATION_ROLES;
+			first = false;
+		} else {
+			mutex_release(&vcpu->mutex.dep_map, _THIS_IP_);
+		}
 	}
 
 	return 0;
 
 out_unlock:
+
+	first = true;
 	kvm_for_each_vcpu(j, vcpu, kvm) {
 		if (i == j)
 			break;
 
+		if (first)
+			first = false;
+		else
+			mutex_acquire(&vcpu->mutex.dep_map, role, 0, _THIS_IP_);
+
+
 		mutex_unlock(&vcpu->mutex);
 	}
 	return -EINTR;
@@ -1621,8 +1648,15 @@
 {
 	struct kvm_vcpu *vcpu;
 	unsigned long i;
+	bool first = true;
 
 	kvm_for_each_vcpu(i, vcpu, kvm) {
+		if (first)
+			first = false;
+		else
+			mutex_acquire(&vcpu->mutex.dep_map,
+				      SEV_NR_MIGRATION_ROLES, 0, _THIS_IP_);
+
 		mutex_unlock(&vcpu->mutex);
 	}
 }
@@ -1748,10 +1782,10 @@
 		charged = true;
 	}
 
-	ret = sev_lock_vcpus_for_migration(kvm);
+	ret = sev_lock_vcpus_for_migration(kvm, SEV_MIGRATION_SOURCE);
 	if (ret)
 		goto out_dst_cgroup;
-	ret = sev_lock_vcpus_for_migration(source_kvm);
+	ret = sev_lock_vcpus_for_migration(source_kvm, SEV_MIGRATION_TARGET);
 	if (ret)
 		goto out_dst_vcpu;