Messages in this thread Patch in this message | | | Date | Mon, 4 Apr 2022 20:35:09 +0000 | From | Sean Christopherson <> | Subject | Re: [PATCH] KVM: SEV: Mark nested locking of vcpu->lock |
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On Mon, Apr 04, 2022, Peter Gonda wrote: > svm_vm_migrate_from() uses sev_lock_vcpus_for_migration() to lock all > source and target vcpu->locks. Mark the nested subclasses to avoid false > positives from lockdep. > > 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> > Signed-off-by: Peter Gonda <pgonda@google.com> > Cc: Paolo Bonzini <pbonzini@redhat.com> > Cc: Sean Christopherson <seanjc@google.com> > Cc: kvm@vger.kernel.org > Cc: linux-kernel@vger.kernel.org > --- > > 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 | 12 +++++++----- > 1 file changed, 7 insertions(+), 5 deletions(-) > > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c > index 75fa6dd268f0..8f77421c1c4b 100644 > --- a/arch/x86/kvm/svm/sev.c > +++ b/arch/x86/kvm/svm/sev.c > @@ -1591,15 +1591,16 @@ static void sev_unlock_two_vms(struct kvm *dst_kvm, struct kvm *src_kvm) > atomic_set_release(&src_sev->migration_in_progress, 0); > } > > - > -static int sev_lock_vcpus_for_migration(struct kvm *kvm) > +static int sev_lock_vcpus_for_migration(struct kvm *kvm, unsigned int *subclass) > { > struct kvm_vcpu *vcpu; > unsigned long i, j; > > kvm_for_each_vcpu(i, vcpu, kvm) { > - if (mutex_lock_killable(&vcpu->mutex)) > + if (mutex_lock_killable_nested(&vcpu->mutex, *subclass)) > goto out_unlock; > + > + ++(*subclass);
This is rather gross, and I'm guessing it adds extra work for the non-lockdep case, assuming the compiler isn't so clever that it can figure out that the result is never used. Not that this is a hot path...
Does each lock actually need a separate subclass? If so, why don't the other paths that lock all vCPUs complain?
If differentiating the two VMs is sufficient, then we can pass in SINGLE_DEPTH_NESTING for the second round of locks. If a per-vCPU subclass is required, we can use the vCPU index and assign evens to one and odds to the other, e.g. this should work and compiles to a nop when LOCKDEP is disabled (compile tested only). It's still gross, but we could pretty it up, e.g. add defines for the 0/1 param.
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c index 75fa6dd268f0..9be35902b809 100644 --- a/arch/x86/kvm/svm/sev.c +++ b/arch/x86/kvm/svm/sev.c @@ -1591,14 +1591,13 @@ static void sev_unlock_two_vms(struct kvm *dst_kvm, struct kvm *src_kvm) atomic_set_release(&src_sev->migration_in_progress, 0); }
- -static int sev_lock_vcpus_for_migration(struct kvm *kvm) +static int sev_lock_vcpus_for_migration(struct kvm *kvm, int mod) { struct kvm_vcpu *vcpu; unsigned long i, j;
kvm_for_each_vcpu(i, vcpu, kvm) { - if (mutex_lock_killable(&vcpu->mutex)) + if (mutex_lock_killable_nested(&vcpu->mutex, i * 2 + mod)) goto out_unlock; }
@@ -1745,10 +1744,10 @@ 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, 0); if (ret) goto out_dst_cgroup; - ret = sev_lock_vcpus_for_migration(source_kvm); + ret = sev_lock_vcpus_for_migration(source_kvm, 1); if (ret) goto out_dst_vcpu;
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