Re: [RFC v3 5/6] qmp: add QMP command x-debug-virtio-queue-element

[Jason Wang <jasowang@redhat.com>]

On 2020/5/7 下午7:49, Laurent Vivier wrote:
> This new command shows the information of a VirtQueue element.
>
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>   hw/virtio/virtio-stub.c |   9 +++
>   hw/virtio/virtio.c      | 130 ++++++++++++++++++++++++++++++++++++++++
>   qapi/virtio.json        |  94 +++++++++++++++++++++++++++++
>   3 files changed, 233 insertions(+)
>
> diff --git a/hw/virtio/virtio-stub.c b/hw/virtio/virtio-stub.c
> index 3c1bf172acf6..8275e31430e7 100644
> --- a/hw/virtio/virtio-stub.c
> +++ b/hw/virtio/virtio-stub.c
> @@ -23,3 +23,12 @@ VirtQueueStatus *qmp_x_debug_virtio_queue_status(const char *path,
>   {
>       return qmp_virtio_unsupported(errp);
>   }
> +
> +VirtioQueueElement *qmp_x_debug_virtio_queue_element(const char* path,
> +                                                     uint16_t queue,
> +                                                     bool has_index,
> +                                                     uint16_t index,
> +                                                     Error **errp)
> +{
> +    return qmp_virtio_unsupported(errp);
> +}
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 57552bf05014..66dc2cef1b39 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -4033,6 +4033,136 @@ VirtioStatus *qmp_x_debug_virtio_status(const char* path, Error **errp)
>       return status;
>   }
>   
> +static VirtioRingDescFlagsList *qmp_decode_vring_desc_flags(uint16_t flags)
> +{
> +    VirtioRingDescFlagsList *list = NULL;
> +    VirtioRingDescFlagsList *node;
> +    int i;
> +    struct {
> +        uint16_t flag;
> +        VirtioRingDescFlags value;
> +    } map[] = {
> +        { VRING_DESC_F_NEXT, VIRTIO_RING_DESC_FLAGS_NEXT },
> +        { VRING_DESC_F_WRITE, VIRTIO_RING_DESC_FLAGS_WRITE },
> +        { VRING_DESC_F_INDIRECT, VIRTIO_RING_DESC_FLAGS_INDIRECT },
> +        { 1 << VRING_PACKED_DESC_F_AVAIL, VIRTIO_RING_DESC_FLAGS_AVAIL },
> +        { 1 << VRING_PACKED_DESC_F_USED, VIRTIO_RING_DESC_FLAGS_USED },
> +        { 0, -1 }
> +    };
> +
> +    for (i = 0; map[i].flag; i++) {
> +        if ((map[i].flag & flags) == 0) {
> +            continue;
> +        }
> +        node = g_malloc0(sizeof(VirtioRingDescFlagsList));
> +        node->value = map[i].value;
> +        node->next = list;
> +        list = node;
> +    }
> +
> +    return list;
> +}
> +
> +VirtioQueueElement *qmp_x_debug_virtio_queue_element(const char* path,
> +                                                     uint16_t queue,
> +                                                     bool has_index,
> +                                                     uint16_t index,
> +                                                     Error **errp)
> +{
> +    VirtIODevice *vdev;
> +    VirtQueue *vq;
> +    VirtioQueueElement *element = NULL;
> +
> +    vdev = virtio_device_find(path);
> +    if (vdev == NULL) {
> +        error_setg(errp, "Path %s is not a VirtIO device", path);
> +        return NULL;
> +    }
> +
> +    if (queue >= VIRTIO_QUEUE_MAX || !virtio_queue_get_num(vdev, queue)) {
> +        error_setg(errp, "Invalid virtqueue number %d", queue);
> +        return NULL;
> +    }
> +    vq = &vdev->vq[queue];
> +
> +    if (virtio_vdev_has_feature(vdev, VIRTIO_F_RING_PACKED)) {
> +        error_setg(errp, "Packed ring not supported");
> +        return NULL;
> +    } else {
> +        unsigned int head, i, max;
> +        VRingMemoryRegionCaches *caches;
> +        MemoryRegionCache indirect_desc_cache = MEMORY_REGION_CACHE_INVALID;
> +        MemoryRegionCache *desc_cache;
> +        VRingDesc desc;
> +        VirtioRingDescList *list = NULL;
> +        VirtioRingDescList *node;
> +        int rc;
> +
> +        RCU_READ_LOCK_GUARD();
> +
> +        max = vq->vring.num;
> +
> +        if (!has_index) {
> +            head = vring_avail_ring(vq, vq->last_avail_idx % vq->vring.num);
> +        } else {
> +            head = vring_avail_ring(vq, index % vq->vring.num);
> +        }
> +        i = head;
> +
> +        caches = vring_get_region_caches(vq);
> +        if (!caches) {
> +            error_setg(errp, "Region caches not initialized");
> +            return NULL;
> +        }
> +
> +        if (caches->desc.len < max * sizeof(VRingDesc)) {
> +            error_setg(errp, "Cannot map descriptor ring");
> +            return NULL;
> +        }
> +
> +        desc_cache = &caches->desc;
> +        vring_split_desc_read(vdev, &desc, desc_cache, i);
> +        if (desc.flags & VRING_DESC_F_INDIRECT) {
> +            int64_t len;
> +
> +            len = address_space_cache_init(&indirect_desc_cache, vdev->dma_as,
> +                                           desc.addr, desc.len, false);
> +            desc_cache = &indirect_desc_cache;
> +            if (len < desc.len) {
> +                error_setg(errp, "Cannot map indirect buffer");
> +                goto done;
> +            }
> +            i = 0;
> +            vring_split_desc_read(vdev, &desc, desc_cache, i);
> +        }
> +
> +        element = g_new0(VirtioQueueElement, 1);
> +        element->index = head;
> +        element->ndescs = 0;
> +
> +        do {
> +            node = g_new0(VirtioRingDescList, 1);
> +            node->value = g_new0(VirtioRingDesc, 1);
> +            node->value->addr = desc.addr;
> +            node->value->len = desc.len;
> +            node->value->flags = qmp_decode_vring_desc_flags(desc.flags);
> +            node->next = list;
> +            list = node;
> +
> +            element->ndescs++;
> +
> +            rc = virtqueue_split_read_next_desc(vdev, &desc, desc_cache,
> +                                                max, &i);
> +        } while (rc == VIRTQUEUE_READ_DESC_MORE);


It's better to limit the maximum number of iterations here to e.g 
vring.num. A buggy driver may produce a infinite loop here.

Thanks


> +
> +        element->descs = list;
> +done:
> +        address_space_cache_destroy(&indirect_desc_cache);
> +    }
> +
> +    return element;
> +}
> +
>   static const TypeInfo virtio_device_info = {
>       .name = TYPE_VIRTIO_DEVICE,
>       .parent = TYPE_DEVICE,
> diff --git a/qapi/virtio.json b/qapi/virtio.json
> index 43c234a9fc69..a55103dca780 100644
> --- a/qapi/virtio.json
> +++ b/qapi/virtio.json
> @@ -406,3 +406,97 @@
>     'data': { 'path': 'str', 'queue': 'uint16' },
>     'returns': 'VirtQueueStatus'
>   }
> +
> +##
> +# @VirtioRingDescFlags:
> +#
> +# An enumeration of the virtio ring descriptor flags
> +#
> +# Since: 5.1
> +#
> +##
> +
> +{ 'enum': 'VirtioRingDescFlags',
> +  'data': [ 'next', 'write', 'indirect', 'avail', 'used' ]
> +}
> +
> +##
> +# @VirtioRingDesc:
> +#
> +# @addr: guest physical address of the descriptor data
> +#
> +# @len: length of the descriptor data
> +#
> +# @flags: descriptor flags
> +#
> +# Since: 5.1
> +#
> +##
> +
> +{ 'struct': 'VirtioRingDesc',
> +  'data': {
> +    'addr': 'uint64',
> +    'len': 'uint32',
> +    'flags': [ 'VirtioRingDescFlags' ]
> +  }
> +}
> +
> +##
> +# @VirtioQueueElement:
> +#
> +# @index: index of the element in the queue
> +#
> +# @ndescs: number of descriptors
> +#
> +# @descs: list of the descriptors
> +#
> +# Since: 5.1
> +#
> +##
> +
> +{ 'struct': 'VirtioQueueElement',
> +  'data': {
> +    'index': 'uint32',
> +    'ndescs': 'uint32',
> +    'descs': ['VirtioRingDesc']
> +  }
> +}
> +
> +##
> +# @x-debug-virtio-queue-element:
> +#
> +# Return the information about an element queue (by default head)
> +#
> +# @path: QOBject path of the VirtIODevice
> +#
> +# @queue: queue number to examine
> +#
> +# @index: the index in the queue, by default head
> +#
> +# Returns: the element information
> +#
> +# Since: 5.1
> +#
> +# Example:
> +#
> +# -> { "execute": "x-debug-virtio-queue-element",
> +#      "arguments": {
> +#          "path": "/machine/peripheral-anon/device[3]/virtio-backend",
> +#          "queue": 0
> +#      }
> +#   }
> +# -> { "return": {
> +#         "index": 24,
> +#         "ndescs": 1,
> +#         "descs": [
> +#             { "flags": ["write"], "len": 1536, "addr": 2027557376 }
> +#         ]
> +#      }
> +#   }
> +#
> +##
> +
> +{ 'command': 'x-debug-virtio-queue-element',
> +  'data': { 'path': 'str', 'queue': 'uint16', '*index': 'uint16' },
> +  'returns': 'VirtioQueueElement'
> +}