Re: [Qemu-devel] [PATCH v4 10/11] virtio: migration support for packed ring

[Wei Xu <wexu@redhat.com>]

On Tue, Feb 19, 2019 at 03:30:41PM +0800, Jason Wang wrote:
> 
> On 2019/2/14 下午12:26, wexu@redhat.com wrote:
> >From: Wei Xu <wexu@redhat.com>
> >
> >Both userspace and vhost-net/user are supported with this patch.
> >
> >A new subsection is introduced for packed ring, only 'last_avail_idx'
> >and 'last_avail_wrap_counter' are saved/loaded presumably based on
> >all the others relevant data(inuse, used/avail index and wrap count
> >should be the same.
> 
> 
> This is probably only true for net device, see comment in virtio_load():
> 
>             /*
>              * Some devices migrate VirtQueueElements that have been popped
>              * from the avail ring but not yet returned to the used ring.
>              * Since max ring size < UINT16_MAX it's safe to use modulo
>              * UINT16_MAX + 1 subtraction.
>              */
>             vdev->vq[i].inuse = (uint16_t)(vdev->vq[i].last_avail_idx -
>                                 vdev->vq[i].used_idx);
> 
> 
> So you need to migrate used_idx and used_wrap_counter since we don't have
> used idx.

This is trying to align with vhost-net/user as we discussed, since all we
have done is to support  virtio-net device for packed ring, maybe we can
consider supporting other devices after we have got it verified.

> 
> 
> >
> >Signed-off-by: Wei Xu <wexu@redhat.com>
> >---
> >  hw/virtio/virtio.c | 69 +++++++++++++++++++++++++++++++++++++++++++++++++++---
> >  1 file changed, 66 insertions(+), 3 deletions(-)
> >
> >diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> >index 8cfc7b6..7c5de07 100644
> >--- a/hw/virtio/virtio.c
> >+++ b/hw/virtio/virtio.c
> >@@ -2349,6 +2349,13 @@ static bool virtio_virtqueue_needed(void *opaque)
> >      return virtio_host_has_feature(vdev, VIRTIO_F_VERSION_1);
> >  }
> >+static bool virtio_packed_virtqueue_needed(void *opaque)
> >+{
> >+    VirtIODevice *vdev = opaque;
> >+
> >+    return virtio_host_has_feature(vdev, VIRTIO_F_RING_PACKED);
> >+}
> >+
> >  static bool virtio_ringsize_needed(void *opaque)
> >  {
> >      VirtIODevice *vdev = opaque;
> >@@ -2390,6 +2397,17 @@ static const VMStateDescription vmstate_virtqueue = {
> >      }
> >  };
> >+static const VMStateDescription vmstate_packed_virtqueue = {
> >+    .name = "packed_virtqueue_state",
> >+    .version_id = 1,
> >+    .minimum_version_id = 1,
> >+    .fields = (VMStateField[]) {
> >+        VMSTATE_UINT16(last_avail_idx, struct VirtQueue),
> >+        VMSTATE_BOOL(last_avail_wrap_counter, struct VirtQueue),
> >+        VMSTATE_END_OF_LIST()
> >+    }
> >+};
> >+
> >  static const VMStateDescription vmstate_virtio_virtqueues = {
> >      .name = "virtio/virtqueues",
> >      .version_id = 1,
> >@@ -2402,6 +2420,18 @@ static const VMStateDescription vmstate_virtio_virtqueues = {
> >      }
> >  };
> >+static const VMStateDescription vmstate_virtio_packed_virtqueues = {
> >+    .name = "virtio/packed_virtqueues",
> >+    .version_id = 1,
> >+    .minimum_version_id = 1,
> >+    .needed = &virtio_packed_virtqueue_needed,
> >+    .fields = (VMStateField[]) {
> >+        VMSTATE_STRUCT_VARRAY_POINTER_KNOWN(vq, struct VirtIODevice,
> >+                      VIRTIO_QUEUE_MAX, 0, vmstate_packed_virtqueue, VirtQueue),
> >+        VMSTATE_END_OF_LIST()
> >+    }
> >+};
> >+
> >  static const VMStateDescription vmstate_ringsize = {
> >      .name = "ringsize_state",
> >      .version_id = 1,
> >@@ -2522,6 +2552,7 @@ static const VMStateDescription vmstate_virtio = {
> >          &vmstate_virtio_ringsize,
> >          &vmstate_virtio_broken,
> >          &vmstate_virtio_extra_state,
> >+        &vmstate_virtio_packed_virtqueues,
> >          NULL
> >      }
> >  };
> >@@ -2794,6 +2825,17 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int version_id)
> >                  virtio_queue_update_rings(vdev, i);
> >              }
> >+            if (virtio_vdev_has_feature(vdev, VIRTIO_F_RING_PACKED)) {
> >+                vdev->vq[i].shadow_avail_idx = vdev->vq[i].last_avail_idx;
> >+                vdev->vq[i].avail_wrap_counter =
> >+                                        vdev->vq[i].last_avail_wrap_counter;
> >+
> >+                vdev->vq[i].used_idx = vdev->vq[i].last_avail_idx;
> >+                vdev->vq[i].used_wrap_counter =
> >+                                        vdev->vq[i].last_avail_wrap_counter;
> >+                continue;
> >+            }
> >+
> >              nheads = vring_avail_idx(&vdev->vq[i]) - vdev->vq[i].last_avail_idx;
> >              /* Check it isn't doing strange things with descriptor numbers. */
> >              if (nheads > vdev->vq[i].vring.num) {
> >@@ -2955,17 +2997,34 @@ hwaddr virtio_queue_get_used_size(VirtIODevice *vdev, int n)
> >  uint16_t virtio_queue_get_last_avail_idx(VirtIODevice *vdev, int n)
> >  {
> >-    return vdev->vq[n].last_avail_idx;
> >+    uint16_t idx;
> >+
> >+    if (virtio_vdev_has_feature(vdev, VIRTIO_F_RING_PACKED)) {
> >+        idx = vdev->vq[n].last_avail_idx;
> >+        idx |= ((int)vdev->vq[n].avail_wrap_counter) << 15;
> >+    } else {
> >+        idx = (int)vdev->vq[n].last_avail_idx;
> >+    }
> >+    return idx;
> >  }
> >  void virtio_queue_set_last_avail_idx(VirtIODevice *vdev, int n, uint16_t idx)
> >  {
> >-    vdev->vq[n].last_avail_idx = idx;
> >-    vdev->vq[n].shadow_avail_idx = idx;
> >+    if (virtio_vdev_has_feature(vdev, VIRTIO_F_RING_PACKED)) {
> >+        vdev->vq[n].last_avail_idx = idx & 0x7fff;
> >+        vdev->vq[n].avail_wrap_counter = !!(idx & 0x8000);
> 
> 
> Let's define some macros for those magic number.

OK.

> 
> 
> >+    } else {
> >+        vdev->vq[n].last_avail_idx = idx;
> >+        vdev->vq[n].shadow_avail_idx = idx;
> >+    }
> >  }
> >  void virtio_queue_restore_last_avail_idx(VirtIODevice *vdev, int n)
> >  {
> >+    if (virtio_vdev_has_feature(vdev, VIRTIO_F_RING_PACKED)) {
> >+        return;
> >+    }
> 
> 
> Why doesn't packed ring care about this?

As elaborated above, used idx/wrap_counter are supposed to be the
same with avail ones for vhost-net/user.

> 
> 
> >+
> >      rcu_read_lock();
> >      if (vdev->vq[n].vring.desc) {
> >          vdev->vq[n].last_avail_idx = vring_used_idx(&vdev->vq[n]);
> >@@ -2976,6 +3035,10 @@ void virtio_queue_restore_last_avail_idx(VirtIODevice *vdev, int n)
> >  void virtio_queue_update_used_idx(VirtIODevice *vdev, int n)
> >  {
> >+    if (virtio_vdev_has_feature(vdev, VIRTIO_F_RING_PACKED)) {
> >+        return;
> >+    }
> 
> 
> And this?

Same as above.

Wei

> 
> Thanks
> 
> 
> >+
> >      rcu_read_lock();
> >      if (vdev->vq[n].vring.desc) {
> >          vdev->vq[n].used_idx = vring_used_idx(&vdev->vq[n]);