Re: [RFC PATCH 3/3] hw/virtio: Have virtqueue_get_avail_bytes() pass caches arg to callees

[Stefano Garzarella]

On Thu, Aug 26, 2021 at 07:26:58PM +0200, Philippe Mathieu-Daudé wrote:
> Both virtqueue_packed_get_avail_bytes() and
> virtqueue_split_get_avail_bytes() access the region cache, but
> their caller also does. Simplify by having virtqueue_get_avail_bytes
> calling both with RCU lock held, and passing the caches as argument.
>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
> RFC because I'm not sure this is safe enough

It seems safe to me.

While reviewing I saw that vring_get_region_caches() has
/* Called within rcu_read_lock().  */ comment, but it seems to me that 
we call that function in places where we haven't acquired it, which 
shouldn't be a problem, but should we remove that comment?

Thanks,
Stefano

> ---
> hw/virtio/virtio.c | 29 ++++++++++++-----------------
> 1 file changed, 12 insertions(+), 17 deletions(-)
>
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 3a1f6c520cb..8237693a567 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -984,28 +984,23 @@ static int virtqueue_split_read_next_desc(VirtIODevice 
> *vdev, VRingDesc *desc,
>     return VIRTQUEUE_READ_DESC_MORE;
> }
>
> +/* Called within rcu_read_lock().  */
> static void virtqueue_split_get_avail_bytes(VirtQueue *vq,
>                             unsigned int *in_bytes, unsigned int *out_bytes,
> -                            unsigned max_in_bytes, unsigned max_out_bytes)
> +                            unsigned max_in_bytes, unsigned max_out_bytes,
> +                            VRingMemoryRegionCaches *caches)
> {
>     VirtIODevice *vdev = vq->vdev;
>     unsigned int max, idx;
>     unsigned int total_bufs, in_total, out_total;
> -    VRingMemoryRegionCaches *caches;
>     MemoryRegionCache indirect_desc_cache = MEMORY_REGION_CACHE_INVALID;
>     int64_t len = 0;
>     int rc;
>
> -    RCU_READ_LOCK_GUARD();
> -
>     idx = vq->last_avail_idx;
>     total_bufs = in_total = out_total = 0;
>
>     max = vq->vring.num;
> -    caches = vring_get_region_caches(vq);
> -    if (!caches) {
> -        goto err;
> -    }
>
>     while ((rc = virtqueue_num_heads(vq, idx)) > 0) {
>         MemoryRegionCache *desc_cache = &caches->desc;
> @@ -1124,32 +1119,28 @@ static int virtqueue_packed_read_next_desc(VirtQueue 
> *vq,
>     return VIRTQUEUE_READ_DESC_MORE;
> }
>
> +/* Called within rcu_read_lock().  */
> static void virtqueue_packed_get_avail_bytes(VirtQueue *vq,
>                                              unsigned int *in_bytes,
>                                              unsigned int *out_bytes,
>                                              unsigned max_in_bytes,
> -                                             unsigned max_out_bytes)
> +                                             unsigned max_out_bytes,
> +                                             VRingMemoryRegionCaches *caches)
> {
>     VirtIODevice *vdev = vq->vdev;
>     unsigned int max, idx;
>     unsigned int total_bufs, in_total, out_total;
>     MemoryRegionCache *desc_cache;
> -    VRingMemoryRegionCaches *caches;
>     MemoryRegionCache indirect_desc_cache = MEMORY_REGION_CACHE_INVALID;
>     int64_t len = 0;
>     VRingPackedDesc desc;
>     bool wrap_counter;
>
> -    RCU_READ_LOCK_GUARD();
>     idx = vq->last_avail_idx;
>     wrap_counter = vq->last_avail_wrap_counter;
>     total_bufs = in_total = out_total = 0;
>
>     max = vq->vring.num;
> -    caches = vring_get_region_caches(vq);
> -    if (!caches) {
> -        goto err;
> -    }
>
>     for (;;) {
>         unsigned int num_bufs = total_bufs;
> @@ -1250,6 +1241,8 @@ void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned 
> int *in_bytes,
>     uint16_t desc_size;
>     VRingMemoryRegionCaches *caches;
>
> +    RCU_READ_LOCK_GUARD();
> +
>     if (unlikely(!vq->vring.desc)) {
>         goto err;
>     }
> @@ -1268,10 +1261,12 @@ void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned 
> int *in_bytes,
>
>     if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED)) {
>         virtqueue_packed_get_avail_bytes(vq, in_bytes, out_bytes,
> -                                         max_in_bytes, max_out_bytes);
> +                                         max_in_bytes, max_out_bytes,
> +                                         caches);
>     } else {
>         virtqueue_split_get_avail_bytes(vq, in_bytes, out_bytes,
> -                                        max_in_bytes, max_out_bytes);
> +                                        max_in_bytes, max_out_bytes,
> +                                        caches);
>     }
>
>     return;
> --
> 2.31.1