[patch 2/4] KVM: move coalesced_mmio locking to its own device

[Marcelo Tosatti <mtosatti@xxxxxxxxxx>]

Get rid of kvm->lock dependency on coalesced_mmio methods. Use an 
atomic variable instead to guarantee only one vcpu is batching
data into the ring at a given time.

Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>

Index: kvm-irqlock/virt/kvm/coalesced_mmio.c
===================================================================
--- kvm-irqlock.orig/virt/kvm/coalesced_mmio.c
+++ kvm-irqlock/virt/kvm/coalesced_mmio.c
@@ -26,9 +26,12 @@ static int coalesced_mmio_in_range(struc
 	if (!is_write)
 		return 0;
 
-	/* kvm->lock is taken by the caller and must be not released before
-         * dev.read/write
-         */
+	/*
+ 	 * Some other vcpu might be batching data into the ring,
+ 	 * fallback to userspace. Ordering not our problem.
+ 	 */
+	if (!atomic_add_unless(&dev->in_use, 1, 1))
+		return 0;
 
 	/* Are we able to batch it ? */
 
@@ -41,7 +44,7 @@ static int coalesced_mmio_in_range(struc
 							KVM_COALESCED_MMIO_MAX;
 	if (next == dev->kvm->coalesced_mmio_ring->first) {
 		/* full */
-		return 0;
+		goto out_denied;
 	}
 
 	/* is it in a batchable area ? */
@@ -57,6 +60,8 @@ static int coalesced_mmio_in_range(struc
 		    addr + len <= zone->addr + zone->size)
 			return 1;
 	}
+out_denied:
+	atomic_set(&dev->in_use, 0);
 	return 0;
 }
 
@@ -67,15 +72,14 @@ static void coalesced_mmio_write(struct 
 				(struct kvm_coalesced_mmio_dev*)this->private;
 	struct kvm_coalesced_mmio_ring *ring = dev->kvm->coalesced_mmio_ring;
 
-	/* kvm->lock must be taken by caller before call to in_range()*/
-
 	/* copy data in first free entry of the ring */
 
 	ring->coalesced_mmio[ring->last].phys_addr = addr;
 	ring->coalesced_mmio[ring->last].len = len;
 	memcpy(ring->coalesced_mmio[ring->last].data, val, len);
-	smp_wmb();
 	ring->last = (ring->last + 1) % KVM_COALESCED_MMIO_MAX;
+	smp_wmb();
+	atomic_set(&dev->in_use, 0);
 }
 
 static void coalesced_mmio_destructor(struct kvm_io_device *this)
@@ -90,6 +94,8 @@ int kvm_coalesced_mmio_init(struct kvm *
 	dev = kzalloc(sizeof(struct kvm_coalesced_mmio_dev), GFP_KERNEL);
 	if (!dev)
 		return -ENOMEM;
+	atomic_set(&dev->in_use, 0);
+
 	dev->dev.write  = coalesced_mmio_write;
 	dev->dev.in_range  = coalesced_mmio_in_range;
 	dev->dev.destructor  = coalesced_mmio_destructor;
Index: kvm-irqlock/virt/kvm/coalesced_mmio.h
===================================================================
--- kvm-irqlock.orig/virt/kvm/coalesced_mmio.h
+++ kvm-irqlock/virt/kvm/coalesced_mmio.h
@@ -12,6 +12,7 @@
 struct kvm_coalesced_mmio_dev {
 	struct kvm_io_device dev;
 	struct kvm *kvm;
+	atomic_t in_use;
 	int nb_zones;
 	struct kvm_coalesced_mmio_zone zone[KVM_COALESCED_MMIO_ZONE_MAX];
 };

-- 

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