Re: [Qemu-devel] [[RFC v3 05/12] virtio: init and desc empty check for packed ring

[Jason Wang]

On 2018年10月11日 22:08, w...@redhat.com wrote:
From: Wei Xu <w...@redhat.com>

Basic initialization and helpers for packed ring.

Signed-off-by: Wei Xu <w...@redhat.com>
---
  hw/virtio/virtio.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++-
  1 file changed, 56 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 9185efb..86f88da 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -24,6 +24,9 @@
  #include "hw/virtio/virtio-access.h"
  #include "sysemu/dma.h"
  
+#define AVAIL_DESC_PACKED(b) ((b) << 7)
+#define USED_DESC_PACKED(b)  ((b) << 15)
+
  /*
   * The alignment to use between consumer and producer parts of vring.
   * x86 pagesize again. This is the default, used by transports like PCI
@@ -372,6 +375,23 @@ int virtio_queue_ready(VirtQueue *vq)
      return vq->vring.avail != 0;
  }
  
+static void vring_packed_desc_read_flags(VirtIODevice *vdev,
+                    VRingPackedDesc *desc, MemoryRegionCache *cache, int i)
+{
+    address_space_read_cached(cache,
+              i * sizeof(VRingPackedDesc) + offsetof(VRingPackedDesc, flags),
+              &desc->flags, sizeof(desc->flags));
+}
+
+static inline bool is_desc_avail(struct VRingPackedDesc *desc, bool wc)
+{

I think it's better use wrap_counter instead of wc here (unless you want 
to use wc everywhere which is a even worse idea).

Thanks

+    bool avail, used;
+
+    avail = !!(desc->flags & AVAIL_DESC_PACKED(1));
+    used = !!(desc->flags & USED_DESC_PACKED(1));
+    return (avail != used) && (avail == wc);
+}
+
  /* Fetch avail_idx from VQ memory only when we really need to know if
   * guest has added some buffers.
   * Called within rcu_read_lock().  */
@@ -392,7 +412,7 @@ static int virtio_queue_empty_rcu(VirtQueue *vq)
      return vring_avail_idx(vq) == vq->last_avail_idx;
  }
  
-int virtio_queue_empty(VirtQueue *vq)
+static int virtio_queue_split_empty(VirtQueue *vq)
  {
      bool empty;
  
@@ -414,6 +434,41 @@ int virtio_queue_empty(VirtQueue *vq)
      return empty;
  }
  
+static int virtio_queue_packed_empty_rcu(VirtQueue *vq)
+{
+    struct VRingPackedDesc desc;
+    VRingMemoryRegionCaches *cache;
+
+    if (unlikely(!vq->vring.desc)) {
+        return 1;
+    }
+
+    cache = vring_get_region_caches(vq);
+    vring_packed_desc_read_flags(vq->vdev, &desc, &cache->desc,
+                                vq->last_avail_idx);
+
+    return !is_desc_avail(&desc, vq->avail_wrap_counter);
+}
+
+static int virtio_queue_packed_empty(VirtQueue *vq)
+{
+    bool empty;
+
+    rcu_read_lock();
+    empty = virtio_queue_packed_empty_rcu(vq);
+    rcu_read_unlock();
+    return empty;
+}
+
+int virtio_queue_empty(VirtQueue *vq)
+{
+    if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED)) {
+        return virtio_queue_packed_empty(vq);
+    } else {
+        return virtio_queue_split_empty(vq);
+    }
+}
+
  static void virtqueue_unmap_sg(VirtQueue *vq, const VirtQueueElement *elem,
                                 unsigned int len)
  {