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Subject[PATCH v4] KVM: SEV: Mark nested locking of vcpu->lock
From
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>

---

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
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