Re: [Qemu-devel] [PATCH 8/8] virtio: queue pop support for packed ring

[Jason Wang]

On 2018年04月04日 20:54, address@hidden wrote:
> From: Wei Xu <address@hidden>
>
> cloned from split ring pop, a global static length array
> and the inside-element length array are introduced to
> easy prototype, this consumes more memory and it is valuable
> to move to dynamic allocation as the out/in sg does.

To ease the reviewer, I suggest to reorder this patch to patch 4.

> Signed-off-by: Wei Xu <address@hidden>
> ---
>   hw/virtio/virtio.c | 154 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 153 insertions(+), 1 deletion(-)
>
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index cf726f3..0eafb38 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -1221,7 +1221,7 @@ static void *virtqueue_alloc_element(size_t sz, unsigned 
> out_num, unsigned in_nu
>       return elem;
>   }
>   
> -void *virtqueue_pop(VirtQueue *vq, size_t sz)
> +static void *virtqueue_pop_split(VirtQueue *vq, size_t sz)
>   {
>       unsigned int i, head, max;
>       VRingMemoryRegionCaches *caches;
> @@ -1356,6 +1356,158 @@ err_undo_map:
>       goto done;
>   }
>   
> +static uint16_t dma_len[VIRTQUEUE_MAX_SIZE];

This looks odd.

> +static void *virtqueue_pop_packed(VirtQueue *vq, size_t sz)
> +{
> +    unsigned int i, head, max;
> +    VRingMemoryRegionCaches *caches;
> +    MemoryRegionCache indirect_desc_cache = MEMORY_REGION_CACHE_INVALID;
> +    MemoryRegionCache *cache;
> +    int64_t len;
> +    VirtIODevice *vdev = vq->vdev;
> +    VirtQueueElement *elem = NULL;
> +    unsigned out_num, in_num, elem_entries;
> +    hwaddr addr[VIRTQUEUE_MAX_SIZE];
> +    struct iovec iov[VIRTQUEUE_MAX_SIZE];
> +    VRingDescPacked desc;
> +    uint8_t wrap_counter;
> +
> +    if (unlikely(vdev->broken)) {
> +        return NULL;
> +    }
> +
> +    vq->last_avail_idx %= vq->packed.num;

Queue size could not be a power of 2.

> +
> +    rcu_read_lock();
> +    if (virtio_queue_empty_packed_rcu(vq)) {
> +        goto done;
> +    }
> +
> +    /* When we start there are none of either input nor output. */
> +    out_num = in_num = elem_entries = 0;
> +
> +    max = vq->vring.num;
> +
> +    if (vq->inuse >= vq->vring.num) {
> +        virtio_error(vdev, "Virtqueue size exceeded");
> +        goto done;
> +    }
> +
> +    if (virtio_vdev_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX)) {
> +        /* FIXME: TBD */
> +    }
> +
> +    head = vq->last_avail_idx;
> +    i = head;
> +
> +    caches = vring_get_region_caches(vq);
> +    cache = &caches->desc_packed;
> +    vring_desc_read_packed(vdev, &desc, cache, i);
> +    if (desc.flags & VRING_DESC_F_INDIRECT) {
> +        if (desc.len % sizeof(VRingDescPacked)) {
> +            virtio_error(vdev, "Invalid size for indirect buffer table");
> +            goto done;
> +        }
> +
> +        /* loop over the indirect descriptor table */
> +        len = address_space_cache_init(&indirect_desc_cache, vdev->dma_as,
> +                                       desc.addr, desc.len, false);
> +        cache = &indirect_desc_cache;
> +        if (len < desc.len) {
> +            virtio_error(vdev, "Cannot map indirect buffer");
> +            goto done;
> +        }
> +
> +        max = desc.len / sizeof(VRingDescPacked);
> +        i = 0;
> +        vring_desc_read_packed(vdev, &desc, cache, i);
> +    }
> +
> +    wrap_counter = vq->wrap_counter;
> +    /* Collect all the descriptors */
> +    while (1) {
> +        bool map_ok;
> +
> +        if (desc.flags & VRING_DESC_F_WRITE) {
> +            map_ok = virtqueue_map_desc(vdev, &in_num, addr + out_num,
> +                                        iov + out_num,
> +                                        VIRTQUEUE_MAX_SIZE - out_num, true,
> +                                        desc.addr, desc.len);
> +        } else {
> +            if (in_num) {
> +                virtio_error(vdev, "Incorrect order for descriptors");
> +                goto err_undo_map;
> +            }
> +            map_ok = virtqueue_map_desc(vdev, &out_num, addr, iov,
> +                                        VIRTQUEUE_MAX_SIZE, false,
> +                                        desc.addr, desc.len);
> +        }
> +        if (!map_ok) {
> +            goto err_undo_map;
> +        }
> +
> +        /* If we've got too many, that implies a descriptor loop. */
> +        if (++elem_entries > max) {
> +            virtio_error(vdev, "Looped descriptor");
> +            goto err_undo_map;
> +        }
> +
> +        dma_len[i++] = desc.len;
> +        /* Toggle wrap_counter for non indirect desc */
> +        if ((i == vq->packed.num) && (cache != &indirect_desc_cache)) {
> +            vq->wrap_counter ^= 1;
> +        }
> +
> +        if (desc.flags & VRING_DESC_F_NEXT) {
> +            vring_desc_read_packed(vq->vdev, &desc, cache, i % vq->packed.num);
> +        } else {
> +            break;
> +        }
> +    }
> +
> +    /* Now copy what we have collected and mapped */
> +    elem = virtqueue_alloc_element(sz, out_num, in_num);
> +    elem->index = head;

This seems wrong, we could not assume buffer id is last_avail_idx.

> +    elem->wrap_counter = wrap_counter;
> +    elem->count = (cache == &indirect_desc_cache) ? 1 : out_num + in_num;
> +    for (i = 0; i < out_num; i++) {
> +        /* DMA Done by marking the length as 0 */

This seems unnecessary, spec said:

"
In a used descriptor, Element Address is unused. Element Length 
specifies the length of the buffer that has
been initialized (written to) by the device.
Element length is reserved for used descriptors without the 
VIRTQ_DESC_F_WRITE flag, and is ignored
by drivers.
"

> +        elem->len[i] = 0;
> +        elem->out_addr[i] = addr[i];
> +        elem->out_sg[i] = iov[i];
> +    }
> +    for (i = 0; i < in_num; i++) {
> +        elem->len[out_num + i] = dma_len[head + out_num + i];

Shouldn't elem->len be determined by virtqueue_fill()?

> +        elem->in_addr[i] = addr[out_num + i];
> +        elem->in_sg[i] = iov[out_num + i];
> +    }
> +
> +    vq->last_avail_idx += elem->count;
> +    vq->last_avail_idx %= vq->packed.num;

Same here, num could not be a power of 2.

> +    vq->inuse += elem->count;
> +
> +    trace_virtqueue_pop(vq, elem, elem->in_num, elem->out_num);
> +done:
> +    address_space_cache_destroy(&indirect_desc_cache);
> +    rcu_read_unlock();
> +
> +    return elem;
> +
> +err_undo_map:
> +    virtqueue_undo_map_desc(out_num, in_num, iov);
> +    g_free(elem);
> +    goto done;

Only few minors difference with split version, can we manage to unify them?

> +}
> +
> +void *virtqueue_pop(VirtQueue *vq, size_t sz)
> +{
> +    if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED)) {
> +        return virtqueue_pop_packed(vq, sz);
> +    } else {
> +        return virtqueue_pop_split(vq, sz);
> +    }
> +}
> +
>   /* virtqueue_drop_all:
>    * @vq: The #VirtQueue
>    * Drops all queued buffers and indicates them to the guest

Thanks