Re: [PATCH v2] KVM: SEV: Mark nested locking of vcpu->mutex

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 05, 2022, Peter Gonda wrote:
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 75fa6dd268f0..673e1ee2cfc9 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -1591,14 +1591,21 @@ static void sev_unlock_two_vms(struct kvm *dst_kvm, struct kvm *src_kvm)
>  	atomic_set_release(&src_sev->migration_in_progress, 0);
>  }
>  
> +#define SEV_MIGRATION_SOURCE 0
> +#define SEV_MIGRATION_TARGET 1
>  
> -static int sev_lock_vcpus_for_migration(struct kvm *kvm)
> +/*
> + * To avoid lockdep warnings callers should pass @vm argument with either

I think it's important to call that these are false positives, saying "avoid
lockdep warnings" suggests we're intentionally not fixing bugs :-)

> + * SEV_MIGRATION_SOURCE or SEV_MIGRATE_TARGET. This allows subclassing of all
> + * vCPU mutex locks.
> + */

If we use an enum, that'll make the param self-documenting.  And we can also use
that to eliminate the remaining magic number '2'.  E.g. this as fixup.

---
 arch/x86/kvm/svm/sev.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 673e1ee2cfc9..1e07d5d3f85a 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -1591,21 +1591,27 @@ static void sev_unlock_two_vms(struct kvm *dst_kvm, struct kvm *src_kvm)
 	atomic_set_release(&src_sev->migration_in_progress, 0);
 }

-#define SEV_MIGRATION_SOURCE 0
-#define SEV_MIGRATION_TARGET 1

 /*
- * To avoid lockdep warnings callers should pass @vm argument with either
- * SEV_MIGRATION_SOURCE or SEV_MIGRATE_TARGET. This allows subclassing of all
- * vCPU mutex locks.
+ * 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.
  */
-static int sev_lock_vcpus_for_migration(struct kvm *kvm, int vm)
+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,
+					enum sev_migration_role role)
 {
 	struct kvm_vcpu *vcpu;
 	unsigned long i, j;

 	kvm_for_each_vcpu(i, vcpu, kvm) {
-		if (mutex_lock_killable_nested(&vcpu->mutex, i * 2 + vm))
+		if (mutex_lock_killable_nested(&vcpu->mutex,
+					       i * SEV_NR_MIGRATION_ROLES + role))
 			goto out_unlock;
 	}


base-commit: 6600ddafa53b35fd5c869aff4a5efb981ed06955
--