[Xen-devel] [PATCHv1 2/2] passthrough: improve locking when iterating over interrupts bound to VMs

[David Vrabel <david.vrabel@xxxxxxxxxx>]

radix_tree_gang_lookup() only requires a RCU read lock, not the
per-domain event_lock.

Introduce a new RCU read lock and take the per-interrupt lock before
calling the callback instead.

This eliminates all contention on the event_lock when injecting
interrupts from passthrough devices.

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
---
 xen/drivers/passthrough/io.c | 10 +++++++---
 xen/include/xen/sched.h      |  1 +
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/xen/drivers/passthrough/io.c b/xen/drivers/passthrough/io.c
index 7c86c20..9b51ef0 100644
--- a/xen/drivers/passthrough/io.c
+++ b/xen/drivers/passthrough/io.c
@@ -601,7 +601,7 @@ int pt_pirq_iterate(struct domain *d,
     unsigned int pirq = 0, n, i;
     struct pirq *pirqs[8];
 
-    ASSERT(spin_is_locked(&d->event_lock));
+    rcu_read_lock(&d->pirq_rcu_lock);
 
     do {
         n = radix_tree_gang_lookup(&d->pirq_tree, (void **)pirqs, pirq,
@@ -610,12 +610,18 @@ int pt_pirq_iterate(struct domain *d,
         {
             struct hvm_pirq_dpci *pirq_dpci = pirq_dpci(pirqs[i]);
 
+            spin_lock(&pirq_dpci->lock);
+
             pirq = pirqs[i]->pirq;
             if ( (pirq_dpci->flags & HVM_IRQ_DPCI_MAPPED) )
                 rc = cb(d, pirq_dpci, arg);
+
+            spin_unlock(&pirq_dpci->lock);
         }
     } while ( !rc && ++pirq < d->nr_pirqs && n == ARRAY_SIZE(pirqs) );
 
+    rcu_read_unlock(&d->pirq_rcu_lock);
+
     return rc;
 }
 
@@ -678,9 +684,7 @@ void hvm_dpci_msi_eoi(struct domain *d, int vector)
     if ( !iommu_enabled || !d->arch.hvm_domain.irq.dpci )
        return;
 
-    spin_lock(&d->event_lock);
     pt_pirq_iterate(d, _hvm_dpci_msi_eoi, (void *)(long)vector);
-    spin_unlock(&d->event_lock);
 }
 
 static void hvm_dirq_assist(struct domain *d, struct hvm_pirq_dpci *pirq_dpci)
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index 3729b0f..ae98c1e 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -355,6 +355,7 @@ struct domain
      */
     struct radix_tree_root pirq_tree;
     unsigned int     nr_pirqs;
+    rcu_read_lock_t pirq_rcu_lock;
 
     enum guest_type guest_type;
 
-- 
2.1.4


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