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

[Wei Xu <wexu@redhat.com>]

On Mon, Oct 15, 2018 at 11:18:05AM +0800, Jason Wang wrote:
> 
> 
> On 2018年10月11日 22:08, wexu@redhat.com wrote:
> >From: Wei Xu <wexu@redhat.com>
> >
> >Basic initialization and helpers for packed ring.
> >
> >Signed-off-by: Wei Xu <wexu@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).

It was to avoid a new line for a parameter since this is a mini function,
I will take it back.

Wei

> 
> 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)
> >  {
>