virtio: Support transports which can specify the vring alignment

Support virtio transports which can specify the vring alignment
(ie where the guest communicates this to the host) by providing
a new virtio_queue_set_align() function. (The default alignment
remains as before.)

Transports which wish to make use of this must set the
has_variable_vring_alignment field in their VirtioBusClass
struct to true; they can then change the alignment via
virtio_queue_set_align().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1373977512-28932-5-git-send-email-peter.maydell@linaro.org

hw/virtio/virtio.c

@@ -19,8 +19,11 @@
 #include "qemu/atomic.h"
 #include "hw/virtio/virtio-bus.h"
 
-/* The alignment to use between consumer and producer parts of vring.
+/*
- * x86 pagesize again. */
+ * The alignment to use between consumer and producer parts of vring.
+ * x86 pagesize again. This is the default, used by transports like PCI
+ * which don't provide a means for the guest to tell the host the alignment.
+ */
 #define VIRTIO_PCI_VRING_ALIGN         4096
 
 typedef struct VRingDesc
@@ -54,6 +57,7 @@ typedef struct VRingUsed
 typedef struct VRing
 {
     unsigned int num;
+    unsigned int align;
     hwaddr desc;
     hwaddr avail;
     hwaddr used;
@@ -93,7 +97,7 @@ static void virtqueue_init(VirtQueue *vq)
     vq->vring.avail = pa + vq->vring.num * sizeof(VRingDesc);
     vq->vring.used = vring_align(vq->vring.avail +
                                  offsetof(VRingAvail, ring[vq->vring.num]),
-                                 VIRTIO_PCI_VRING_ALIGN);
+                                 vq->vring.align);
 }
 
 static inline uint64_t vring_desc_addr(hwaddr desc_pa, int i)
@@ -687,6 +691,21 @@ int virtio_queue_get_id(VirtQueue *vq)
     return vq - &vdev->vq[0];
 }
 
+void virtio_queue_set_align(VirtIODevice *vdev, int n, int align)
+{
+    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
+    VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
+
+    /* Check that the transport told us it was going to do this
+     * (so a buggy transport will immediately assert rather than
+     * silently failing to migrate this state)
+     */
+    assert(k->has_variable_vring_alignment);
+
+    vdev->vq[n].vring.align = align;
+    virtqueue_init(&vdev->vq[n]);
+}
+
 void virtio_queue_notify_vq(VirtQueue *vq)
 {
     if (vq->vring.desc) {
@@ -727,6 +746,7 @@ VirtQueue *virtio_add_queue(VirtIODevice *vdev, int queue_size,
         abort();
 
     vdev->vq[i].vring.num = queue_size;
+    vdev->vq[i].vring.align = VIRTIO_PCI_VRING_ALIGN;
     vdev->vq[i].handle_output = handle_output;
 
     return &vdev->vq[i];
@@ -833,6 +853,9 @@ void virtio_save(VirtIODevice *vdev, QEMUFile *f)
             break;
 
         qemu_put_be32(f, vdev->vq[i].vring.num);
+        if (k->has_variable_vring_alignment) {
+            qemu_put_be32(f, vdev->vq[i].vring.align);
+        }
         qemu_put_be64(f, vdev->vq[i].pa);
         qemu_put_be16s(f, &vdev->vq[i].last_avail_idx);
         if (k->save_queue) {
@@ -889,6 +912,9 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f)
 
     for (i = 0; i < num; i++) {
         vdev->vq[i].vring.num = qemu_get_be32(f);
+        if (k->has_variable_vring_alignment) {
+            vdev->vq[i].vring.align = qemu_get_be32(f);
+        }
         vdev->vq[i].pa = qemu_get_be64(f);
         qemu_get_be16s(f, &vdev->vq[i].last_avail_idx);
         vdev->vq[i].signalled_used_valid = false;

include/hw/virtio/virtio-bus.h

@@ -62,6 +62,12 @@ typedef struct VirtioBusClass {
      * This is called by virtio-bus just before the device is unplugged.
      */
     void (*device_unplug)(DeviceState *d);
+    /*
+     * Does the transport have variable vring alignment?
+     * (ie can it ever call virtio_queue_set_align()?)
+     * Note that changing this will break migration for this transport.
+     */
+    bool has_variable_vring_alignment;
 } VirtioBusClass;
 
 struct VirtioBusState {

include/hw/virtio/virtio.h

@@ -202,6 +202,7 @@ void virtio_queue_set_addr(VirtIODevice *vdev, int n, hwaddr addr);
 hwaddr virtio_queue_get_addr(VirtIODevice *vdev, int n);
 void virtio_queue_set_num(VirtIODevice *vdev, int n, int num);
 int virtio_queue_get_num(VirtIODevice *vdev, int n);
+void virtio_queue_set_align(VirtIODevice *vdev, int n, int align);
 void virtio_queue_notify(VirtIODevice *vdev, int n);
 uint16_t virtio_queue_vector(VirtIODevice *vdev, int n);
 void virtio_queue_set_vector(VirtIODevice *vdev, int n, uint16_t vector);