Re: [PATCH RFC 14/15] hw/virtio/vhost-user: handle data movement with shadow vqs

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Jul 23, 2026 at 03:30:13PM -0700, Connor Kite wrote:
> - Add start logic for shadow virtqueues, which sets vring addresses.
> - Update logic for sending vring addresses to backend to point
>   to the shadow vrings when isolation mode is active.
> - Implement handlers for intercepted avail and used descriptors. These
>   handlers copy buffer contents between bounce buffers in the isolation
>   region and the buffers made available by the guest
> - Implement logic to stop svqs
> 
> Signed-off-by: Connor Kite <connorkite@gmail.com>
> ---
>  hw/virtio/vhost-user.c | 167 ++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 166 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index 75858289a2..e65f877f9a 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -1299,6 +1299,97 @@ static int init_isolation_regions(struct vhost_dev *dev,
>      return 0;
>  }
>  
> +static int vhost_user_memory_lookup(struct vhost_dev *dev, hwaddr gpa,
> +                                    hwaddr *hva)
> +{
> +    int i;
> +    hwaddr offset;
> +
> +    for (i = 0; i < dev->mem->nregions; i++) {
> +        struct vhost_memory_region *reg = dev->mem->regions + i;
> +
> +        if (gpa >= reg->guest_phys_addr &&
> +            reg->guest_phys_addr + reg->memory_size > gpa) {
> +            offset = gpa - reg->guest_phys_addr;
> +            *hva = reg->userspace_addr + offset;
> +            return 0;
> +        }
> +    }
> +
> +    return -EFAULT;
> +}
> +
> +static int vhost_user_svq_handle_used(VhostShadowVirtqueue *svq,
> +                                      VirtQueueElement *elem,
> +                                      void *opaque)
> +{
> +    hwaddr hva;
> +    int r;
> +    struct vhost_dev *dev = opaque;
> +
> +    for (int i = 0; i < elem->in_num; i++) {
> +        r = vhost_user_memory_lookup(dev, elem->in_addr[i], &hva);
> +        if (r < 0) {
> +            return r;
> +        }
> +
> +        memcpy((void *) hva, elem->in_sg[i].iov_base, elem->in_sg[i].iov_len);
> +        elem->in_sg[i].iov_base = (void *) hva;
> +    }
> +
> +    for (int i = 0; i < elem->out_num; i++) {
> +        r = vhost_user_memory_lookup(dev, elem->out_addr[i], &hva);
> +        if (r < 0) {
> +            return r;
> +        }
> +
> +        memcpy((void *) hva, elem->out_sg[i].iov_base, elem->out_sg[i].iov_len);
> +        elem->out_sg[i].iov_base = (void *) hva;
> +    }
> +
> +    return 0;
> +}
> +
> +static int vhost_user_svq_handle_avail(VhostShadowVirtqueue *svq,
> +                                       VirtQueueElement *elem,
> +                                       void *opaque)
> +{
> +    hwaddr offset;
> +    const DMAMap *map;
> +    DMAMap needle;
> +    hwaddr *iova_base;
> +
> +    for (int i = 0; i < elem->out_num; i++) {
> +        needle.translated_addr = elem->out_addr[i];
> +        needle.size = elem->out_sg[i].iov_len - 1;
> +        map = vhost_iova_tree_find_gpa(svq->iova_tree, &needle);

Mapping failure must be handled.

> +        offset = needle.translated_addr - map->translated_addr;
> +        iova_base = (void *)(map->iova + offset);
> +
> +        elem->out_sg[i].iov_base = iova_base;
> +        memcpy(iova_base, elem->out_sg[i].iov_base, needle.size + 1);

iova_base is an iova, not QEMU memory (HVA). Memcpy cannot be used with
IOVAs. The address of the mapped shared memory is needed as the
destination instead.

> +    }
> +
> +    for (int i = 0; i < elem->in_num; i++) {
> +        needle.translated_addr = elem->in_addr[i];
> +        needle.size = elem->in_sg[i].iov_len - 1;
> +        map = vhost_iova_tree_find_gpa(svq->iova_tree, &needle);

Mapping failure must be handled.

> +        offset = needle.translated_addr - map->translated_addr;
> +        iova_base = (void *)(map->iova + offset);
> +
> +        elem->in_sg[i].iov_base = iova_base;
> +        memcpy(iova_base, elem->in_sg[i].iov_base, needle.size + 1);

No memcpy is necessary before vhost_svq_add() since this is data that
will be read from the device upon I/O completion.

> +    }

elem->out_sg[] and elem->in_sg[] are modified in this function. Have we
lost the original I/O buffer memory address from the guest virtqueue?
This is a problem because they will be needed when completing the
request.

> +
> +    vhost_svq_add(svq, elem->out_sg, elem->out_num, elem->out_addr,
> +                  elem->in_sg, elem->in_num, elem->in_addr, elem);
> +
> +    return 0;
> +}
> +
> +
> +
> +
>  static int vhost_user_set_mem_table(struct vhost_dev *dev,
>                                      struct vhost_memory *mem)
>  {
> @@ -1772,6 +1863,8 @@ static int vhost_user_set_vring_err(struct vhost_dev *dev,
>  static int vhost_user_set_vring_addr(struct vhost_dev *dev,
>                                       struct vhost_vring_addr *addr)
>  {
> +    struct vhost_user *u = dev->opaque;
> +
>      VhostUserMsg msg = {
>          .hdr.request = VHOST_USER_SET_VRING_ADDR,
>          .hdr.flags = VHOST_USER_VERSION,
> @@ -1779,6 +1872,25 @@ static int vhost_user_set_vring_addr(struct vhost_dev *dev,
>          .hdr.size = sizeof(msg.payload.addr),
>      };
>  
> +    if (u->user->memory_isolation) {
> +        if (!u->svqs_allocated) {
> +            return 0;
> +        }

Is the idea that this returns silently when called before
vhost_user_dev_start()? State make code harder to understand. It would
be cleaner to set up the vring addresses without relying on
svqs_allocated. What is the reason for deferring the vring address
setup to vhost_user_dev_start()?

> +
> +        int svq_idx = addr->index - dev->vq_index;
> +        VhostShadowVirtqueue *svq = g_ptr_array_index(u->shadow_vqs,
> +                                                      svq_idx);
> +
> +        struct vhost_vring_addr svq_addr = {
> +            .avail_user_addr = (uint64_t)(uintptr_t)svq->vring.avail,
> +            .desc_user_addr = (uint64_t)(uintptr_t)svq->vring.desc,
> +            .used_user_addr = (uint64_t)(uintptr_t)svq->vring.used,
> +            .index = addr->index,
> +        };
> +
> +        msg.payload.addr = svq_addr;
> +    }
> +
>      /*
>       * wait for a reply if logging is enabled to make sure
>       * backend is actually logging changes
> @@ -2754,13 +2866,18 @@ static int vhost_user_postcopy_notifier(NotifierWithReturn *notifier,
>      return 0;
>  }
>  
> +static const VhostShadowVirtqueueOps vhost_user_svq_ops = {
> +    .avail_handler = vhost_user_svq_handle_avail,
> +    .used_handler = vhost_user_svq_handle_used
> +};
> +
>  static void vhost_user_init_svq(struct vhost_dev *dev, struct vhost_user *u)
>  {
>      /*Modified from vhost-vdpa*/
>      u->shadow_vqs = g_ptr_array_new_full(dev->nvqs, vhost_svq_free);
>      for (int i = 0; i < dev->nvqs; i++) {
>          VhostShadowVirtqueue *svq;
> -        svq = vhost_svq_new(NULL, NULL);
> +        svq = vhost_svq_new(&vhost_user_svq_ops, dev);
>          g_ptr_array_add(u->shadow_vqs, svq);
>      }
>  }
> @@ -3466,8 +3583,56 @@ void vhost_user_async_close(DeviceState *d,
>      }
>  }
>  
> +static bool vhost_user_svqs_start(struct vhost_dev *dev)
> +{
> +    struct vhost_user *u = dev->opaque;
> +    uint64_t vring_base = u->iso_memory.vring_base_addr;
> +    u->svqs_allocated = true;

Where is this field cleared to false on reset?

> +
> +    for (int i = 0; i < u->shadow_vqs->len; i++) {
> +        VirtQueue *vq = virtio_get_queue(dev->vdev, dev->vq_index + i);
> +        VhostShadowVirtqueue *svq = g_ptr_array_index(u->shadow_vqs, i);
> +        svq->base_addr = (hwaddr *) vring_base;

Does this support multiple virtqueues, it looks like they will all use
the same base_addr?

> +        vhost_svq_start(svq, dev->vdev, vq, u->iso_iova_tree);
> +
> +        struct vhost_vring_addr addr = {
> +            .index = dev->vq_index + i,
> +            .desc_user_addr = vring_base,
> +            .avail_user_addr = vring_base + sizeof(vring_desc_t) *
> +                svq->vring.num,
> +            .used_user_addr = vring_base + vhost_svq_driver_area_size(svq)
> +        };
> +
> +        vhost_user_set_vring_addr(dev, &addr);

Also mentioned above in vhost_user_set_vring_addr():

Sending a vhost-user message here is strange since that should already
be done by vhost.c:vhost_virtqueue_start() ->
vhost_user_set_vring_addr().

> +
> +        vring_base += vhost_svq_device_area_size(svq) +
> +                      vhost_svq_driver_area_size(svq);
> +    }
> +
> +    return false;
> +}
> +
> +static void vhost_user_svqs_stop(struct vhost_dev *dev)
> +{
> +    struct vhost_user *u = dev->opaque;
> +
> +    for (int i = 0; i < u->shadow_vqs->len; i++) {
> +        vhost_svq_stop(g_ptr_array_index(u->shadow_vqs, i));
> +    }
> +}
> +
> +
>  static int vhost_user_dev_start(struct vhost_dev *dev, bool started)
>  {
> +    struct vhost_user *u = dev->opaque;
> +    if (u->user->memory_isolation) {
> +        if (started) {
> +            vhost_user_svqs_start(dev);
> +        } else {
> +            vhost_user_svqs_stop(dev);
> +        }
> +    }
> +
>      if (!vhost_user_has_protocol_feature(dev, VHOST_USER_PROTOCOL_F_STATUS)) {
>          return 0;
>      }
> 
> -- 
> 2.43.0
> 

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