linux-kernel - [PATCH v4] KVM: SEV: Mark nested locking of vcpu->lock

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Message-Id: <20220502165807.529624-1-pgonda@google.com>
Date:   Mon,  2 May 2022 09:58:07 -0700
From:   Peter Gonda <pgonda@...gle.com>
To:     kvm@...r.kernel.org
Cc:     Peter Gonda <pgonda@...gle.com>,
        John Sperbeck <jsperbeck@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Hillf Danton <hdanton@...a.com>, linux-kernel@...r.kernel.org
Subject: [PATCH v4] 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@...gle.com>
Suggested-by: David Rientjes <rientjes@...gle.com>
Suggested-by: Sean Christopherson <seanjc@...gle.com>
Suggested-by: Paolo Bonzini <pbonzini@...hat.com>
Cc: Hillf Danton <hdanton@...a.com>
Cc: kvm@...r.kernel.org
Cc: linux-kernel@...r.kernel.org
Signed-off-by: Peter Gonda <pgonda@...gle.com>

---

V4
 * Due to 8 subclass limit keep dep_map on only the first vcpu and
   release the others.

V3
 * Updated signature to enum to self-document argument.
 * Updated comment as Seanjc@ suggested.

Tested by running sev_migrate_tests with lockdep enabled. Before we see
a warning from sev_lock_vcpus_for_migration(). After we get no warnings.

---
 arch/x86/kvm/svm/sev.c | 46 ++++++++++++++++++++++++++++++++++++++----
 1 file changed, 42 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 75fa6dd268f0..0239def64eaa 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -1591,24 +1591,55 @@ static void sev_unlock_two_vms(struct kvm *dst_kvm, struct kvm *src_kvm)
 	atomic_set_release(&src_sev->migration_in_progress, 0);
 }
 
+/*
+ * To suppress lockdep false positives, subclass all vCPU mutex locks by
+ * assigning even numbers to the source vCPUs and odd numbers to destination
+ * vCPUs based on the vCPU's index.
+ */
+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;
@@ -1618,8 +1649,15 @@ static void sev_unlock_vcpus_for_migration(struct kvm *kvm)
 {
 	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);
 	}
 }
@@ -1745,10 +1783,10 @@ int sev_vm_move_enc_context_from(struct kvm *kvm, unsigned int source_fd)
 		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;
 
-- 
2.36.0.464.gb9c8b46e94-goog