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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: Wed, 10 Feb 2021 13:54:48 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 2021/2/9 下午11:35, Eugenio Perez Martin wrote:
On Fri, Feb 5, 2021 at 4:52 AM Jason Wang <jasowang@redhat.com> wrote:

On 2021/2/4 下午5:25, Eugenio Perez Martin wrote:
On Thu, Feb 4, 2021 at 4:14 AM Jason Wang <jasowang@redhat.com> wrote:
On 2021/2/2 下午6:17, Eugenio Perez Martin wrote:
On Tue, Feb 2, 2021 at 4:31 AM Jason Wang <jasowang@redhat.com> wrote:
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?

As far as I can tell, we will need the virtio layer the moment we
start copying/translating buffers.

In this series, the virtio dependency can be reduced if qemu does not
check the used ring _F_NO_NOTIFY flag before writing to irqfd. It
would enable packed queues and IOMMU immediately, and I think the cost
should not be so high. In the previous RFC this check was deleted
later anyway, so I think it was a bad idea to include it from the start.
I am not sure I understand here. For vhost, we can still do anything we
want, e.g accessing guest memory etc. Any blocker that prevent us from
copying/translating buffers? (Note that qemu will propagate memory
mappings to vhost).

There is nothing that forbids us to access directly, but if we don't
reuse the virtio layer functionality we would have to duplicate every
access function. "Need" was a too strong word maybe :).

In other words: for the shadow vq vring exposed for the device, qemu
treats it as a driver, and this functionality needs to be added to
qemu. But for accessing the guest's one do not reuse virtio.c would be
a bad idea in my opinion.

The problem is, virtio.c is not a library and it has a lot of dependency
with other qemu modules basically makes it impossible to be reused at
vhost level.

While virtio.c as a whole has dependencies, I think that the functions
needed in the original RFC do not have these dependencies.

However I see how to split vring dataplane from virtio device
management can benefit.

If you can split them out, that would be fine.

We can solve this by:

1) split the core functions out as a library or
2) switch to use contrib/lib-vhostuser but needs to decouple UNIX socket

None of the above looks trivial and they are only device codes. For
shadow virtqueue, we need driver codes as well where no code can be reused.

As we discussed, we probably need IOVA allocated when forwarding
descriptors between the two virtqueues. So my feeling is we can have our
own codes to start then we can consider whether we can reuse some from
the existing virtio.c or lib-vhostuser.

As I see it, if we develop our own code a lot of it will be copied
from current virtio.c, which itself duplicates a lot of contrib/ lib

Maybe it's better to combine your proposals and decouple the vring
functions, the vhost transport, and the qemu virtio device management,
so other projects can reuse them directly?

I think this can work.

I still think this can be left for a later series with buffer
forwarding on top of this one, do you think they can/should be merged

Since you post a new series, let's see.






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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