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Re: [RFC 05/10] vhost: Add vhost_dev_from_virtio

From: Jason Wang
Subject: Re: [RFC 05/10] vhost: Add vhost_dev_from_virtio
Date: Tue, 2 Feb 2021 11:31:38 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 2021/2/1 下午4:28, Eugenio Perez Martin wrote:
On Mon, Feb 1, 2021 at 7:13 AM Jason Wang <jasowang@redhat.com> wrote:

On 2021/1/30 上午4:54, Eugenio Pérez wrote:
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
   include/hw/virtio/vhost.h |  1 +
   hw/virtio/vhost.c         | 17 +++++++++++++++++
   2 files changed, 18 insertions(+)

diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index 4a8bc75415..fca076e3f0 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h
@@ -123,6 +123,7 @@ uint64_t vhost_get_features(struct vhost_dev *hdev, const 
int *feature_bits,
   void vhost_ack_features(struct vhost_dev *hdev, const int *feature_bits,
                           uint64_t features);
   bool vhost_has_free_slot(void);
+struct vhost_dev *vhost_dev_from_virtio(const VirtIODevice *vdev);

   int vhost_net_set_backend(struct vhost_dev *hdev,
                             struct vhost_vring_file *file);
diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index 28c7d78172..8683d507f5 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -61,6 +61,23 @@ bool vhost_has_free_slot(void)
       return slots_limit > used_memslots;

+ * Get the vhost device associated to a VirtIO device.
+ */
+struct vhost_dev *vhost_dev_from_virtio(const VirtIODevice *vdev)
+    struct vhost_dev *hdev;
+    QLIST_FOREACH(hdev, &vhost_devices, entry) {
+        if (hdev->vdev == vdev) {
+            return hdev;
+        }
+    }
+    assert(hdev);
+    return NULL;

I'm not sure this can work in the case of multiqueue. E.g vhost-net
multiqueue is a N:1 mapping between vhost devics and virtio devices.


Right. We could add an "vdev vq index" parameter to the function in
this case, but I guess the most reliable way to do this is to add a
vhost_opaque value to VirtQueue, as Stefan proposed in previous RFC.

So the question still, it looks like it's easier to hide the shadow virtqueue stuffs at vhost layer instead of expose them to virtio layer:

1) vhost protocol is stable ABI
2) no need to deal with virtio stuffs which is more complex than vhost

Or are there any advantages if we do it at virtio layer?


I need to take this into account in qmp_x_vhost_enable_shadow_vq too.

   static void vhost_dev_sync_region(struct vhost_dev *dev,
                                     MemoryRegionSection *section,
                                     uint64_t mfirst, uint64_t mlast,

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