virtio: access ISR atomically

This will be needed once dataplane will be able to set it outside
the big QEMU lock.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-by: Farhan Ali <alifm@linux.vnet.ibm.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio-mmio.c

@@ -191,7 +191,7 @@ static uint64_t virtio_mmio_read(void *opaque, hwaddr offset, unsigned size)
         return virtio_queue_get_addr(vdev, vdev->queue_sel)
             >> proxy->guest_page_shift;
     case VIRTIO_MMIO_INTERRUPTSTATUS:
-        return vdev->isr;
+        return atomic_read(&vdev->isr);
     case VIRTIO_MMIO_STATUS:
         return vdev->status;
     case VIRTIO_MMIO_HOSTFEATURESSEL:
@@ -299,7 +299,7 @@ static void virtio_mmio_write(void *opaque, hwaddr offset, uint64_t value,
         }
         break;
     case VIRTIO_MMIO_INTERRUPTACK:
-        vdev->isr &= ~value;
+        atomic_and(&vdev->isr, ~value);
         virtio_update_irq(vdev);
         break;
     case VIRTIO_MMIO_STATUS:
@@ -347,7 +347,7 @@ static void virtio_mmio_update_irq(DeviceState *opaque, uint16_t vector)
     if (!vdev) {
         return;
     }
-    level = (vdev->isr != 0);
+    level = (atomic_read(&vdev->isr) != 0);
     DPRINTF("virtio_mmio setting IRQ %d\n", level);
     qemu_set_irq(proxy->irq, level);
 }

hw/virtio/virtio-pci.c

@@ -73,7 +73,7 @@ static void virtio_pci_notify(DeviceState *d, uint16_t vector)
         msix_notify(&proxy->pci_dev, vector);
     else {
         VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
-        pci_set_irq(&proxy->pci_dev, vdev->isr & 1);
+        pci_set_irq(&proxy->pci_dev, atomic_read(&vdev->isr) & 1);
     }
 }
 
@@ -449,8 +449,7 @@ static uint32_t virtio_ioport_read(VirtIOPCIProxy *proxy, uint32_t addr)
         break;
     case VIRTIO_PCI_ISR:
         /* reading from the ISR also clears it. */
-        ret = vdev->isr;
+        ret = atomic_xchg(&vdev->isr, 0);
-        vdev->isr = 0;
         pci_irq_deassert(&proxy->pci_dev);
         break;
     case VIRTIO_MSI_CONFIG_VECTOR:
@@ -1379,9 +1378,7 @@ static uint64_t virtio_pci_isr_read(void *opaque, hwaddr addr,
 {
     VirtIOPCIProxy *proxy = opaque;
     VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
-    uint64_t val = vdev->isr;
+    uint64_t val = atomic_xchg(&vdev->isr, 0);
-
-    vdev->isr = 0;
     pci_irq_deassert(&proxy->pci_dev);
 
     return val;

hw/virtio/virtio.c

@@ -945,7 +945,7 @@ void virtio_reset(void *opaque)
     vdev->guest_features = 0;
     vdev->queue_sel = 0;
     vdev->status = 0;
-    vdev->isr = 0;
+    atomic_set(&vdev->isr, 0);
     vdev->config_vector = VIRTIO_NO_VECTOR;
     virtio_notify_vector(vdev, vdev->config_vector);
 
@@ -1318,10 +1318,22 @@ void virtio_del_queue(VirtIODevice *vdev, int n)
     vdev->vq[n].vring.num_default = 0;
 }
 
+static void virtio_set_isr(VirtIODevice *vdev, int value)
+{
+    uint8_t old = atomic_read(&vdev->isr);
+
+    /* Do not write ISR if it does not change, so that its cacheline remains
+     * shared in the common case where the guest does not read it.
+     */
+    if ((old & value) != value) {
+        atomic_or(&vdev->isr, value);
+    }
+}
+
 void virtio_irq(VirtQueue *vq)
 {
     trace_virtio_irq(vq);
-    vq->vdev->isr |= 0x01;
+    virtio_set_isr(vq->vdev, 0x1);
     virtio_notify_vector(vq->vdev, vq->vector);
 }
 
@@ -1355,7 +1367,7 @@ void virtio_notify(VirtIODevice *vdev, VirtQueue *vq)
     }
 
     trace_virtio_notify(vdev, vq);
-    vdev->isr |= 0x01;
+    virtio_set_isr(vq->vdev, 0x1);
     virtio_notify_vector(vdev, vq->vector);
 }
 
@@ -1364,7 +1376,7 @@ void virtio_notify_config(VirtIODevice *vdev)
     if (!(vdev->status & VIRTIO_CONFIG_S_DRIVER_OK))
         return;
 
-    vdev->isr |= 0x03;
+    virtio_set_isr(vdev, 0x3);
     vdev->generation++;
     virtio_notify_vector(vdev, vdev->config_vector);
 }
@@ -1895,7 +1907,7 @@ void virtio_init(VirtIODevice *vdev, const char *name,
 
     vdev->device_id = device_id;
     vdev->status = 0;
-    vdev->isr = 0;
+    atomic_set(&vdev->isr, 0);
     vdev->queue_sel = 0;
     vdev->config_vector = VIRTIO_NO_VECTOR;
     vdev->vq = g_malloc0(sizeof(VirtQueue) * VIRTIO_QUEUE_MAX);