qmp: add QMP command x-query-virtio-queue-element

This new command shows the information of a VirtQueue element.

[Note: Up until v10 of this patch series, virtio.json had many (15+)
 enums defined (e.g. decoded device features, statuses, etc.). In v10
 most of these enums were removed and replaced with string literals.
 By doing this we get (1) simpler schema, (2) smaller generated code,
 and (3) less maintenance burden for when new things are added (e.g.
 devices, device features, etc.).]

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Jonah Palmer <jonah.palmer@oracle.com>
Message-Id: <1660220684-24909-6-git-send-email-jonah.palmer@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

2 files changed, 163 insertions, 0 deletions

diff --git a/hw/virtio/virtio-stub.c b/hw/virtio/virtio-stub.c
index 13e5f93652..7ddb22cc5e 100644
--- a/hw/virtio/virtio-stub.c
+++ b/hw/virtio/virtio-stub.c
@@ -31,3 +31,12 @@ VirtQueueStatus *qmp_x_query_virtio_queue_status(const char *path,
 {
     return qmp_virtio_unsupported(errp);
 }
+
+VirtioQueueElement *qmp_x_query_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 da869e7b51..808446b4c9 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -823,6 +823,19 @@ static inline void vring_used_write(VirtQueue *vq, VRingUsedElem *uelem,
     address_space_cache_invalidate(&caches->used, pa, sizeof(VRingUsedElem));
 }
 
+/* Called within rcu_read_lock(). */
+static inline uint16_t vring_used_flags(VirtQueue *vq)
+{
+    VRingMemoryRegionCaches *caches = vring_get_region_caches(vq);
+    hwaddr pa = offsetof(VRingUsed, flags);
+
+    if (!caches) {
+        return 0;
+    }
+
+    return virtio_lduw_phys_cached(vq->vdev, &caches->used, pa);
+}
+
 /* Called within rcu_read_lock().  */
 static uint16_t vring_used_idx(VirtQueue *vq)
 {
@@ -4782,6 +4795,147 @@ VirtQueueStatus *qmp_x_query_virtio_queue_status(const char *path,
     return status;
 }
 
+static strList *qmp_decode_vring_desc_flags(uint16_t flags)
+{
+    strList *list = NULL;
+    strList *node;
+    int i;
+
+    struct {
+        uint16_t flag;
+        const char *value;
+    } map[] = {
+        { VRING_DESC_F_NEXT, "next" },
+        { VRING_DESC_F_WRITE, "write" },
+        { VRING_DESC_F_INDIRECT, "indirect" },
+        { 1 << VRING_PACKED_DESC_F_AVAIL, "avail" },
+        { 1 << VRING_PACKED_DESC_F_USED, "used" },
+        { 0, "" }
+    };
+
+    for (i = 0; map[i].flag; i++) {
+        if ((map[i].flag & flags) == 0) {
+            continue;
+        }
+        node = g_malloc0(sizeof(strList));
+        node->value = g_strdup(map[i].value);
+        node->next = list;
+        list = node;
+    }
+
+    return list;
+}
+
+VirtioQueueElement *qmp_x_query_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; int ndescs;
+
+        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;
+            }
+
+            max = desc.len / sizeof(VRingDesc);
+            i = 0;
+            vring_split_desc_read(vdev, &desc, desc_cache, i);
+        }
+
+        element = g_new0(VirtioQueueElement, 1);
+        element->avail = g_new0(VirtioRingAvail, 1);
+        element->used = g_new0(VirtioRingUsed, 1);
+        element->name = g_strdup(vdev->name);
+        element->index = head;
+        element->avail->flags = vring_avail_flags(vq);
+        element->avail->idx = vring_avail_idx(vq);
+        element->avail->ring = head;
+        element->used->flags = vring_used_flags(vq);
+        element->used->idx = vring_used_idx(vq);
+        ndescs = 0;
+
+        do {
+            /* A buggy driver may produce an infinite loop */
+            if (ndescs >= max) {
+                break;
+            }
+            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;
+
+            ndescs++;
+            rc = virtqueue_split_read_next_desc(vdev, &desc, desc_cache,
+                                                max, &i);
+        } while (rc == VIRTQUEUE_READ_DESC_MORE);
+        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,