Re: [Qemu-devel] [PATCH v3 1/3] qapi: add x-debug-query-block-graph

[Max Reitz]

On 23.08.18 17:46, Vladimir Sementsov-Ogievskiy wrote:
> Add a new command, returning block nodes (and their users) graph.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> ---
>  qapi/block-core.json           |  91 +++++++++++++++++++++++
>  include/block/block.h          |   1 +
>  include/sysemu/block-backend.h |   2 +
>  block.c                        | 129 +++++++++++++++++++++++++++++++++
>  block/block-backend.c          |   5 ++
>  blockdev.c                     |   5 ++
>  6 files changed, 233 insertions(+)

The design looks better (that is to say, good) to me, so I mostly have
technical remarks.  (Only a bit of bike-shedding this time.)

> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index 4c7a37afdc..34cdc595d7 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -1629,6 +1629,97 @@
>  ##
>  { 'command': 'query-named-block-nodes', 'returns': [ 'BlockDeviceInfo' ] }
>  
> +##
> +# @BlockGraphNodeType:
> +#
> +# Since: 3.1
> +##
> +{ 'enum': 'BlockGraphNodeType',
> +  'data': [ 'blk', 'job', 'bds' ] }

I wouldn't use abbreviations here, but the full names.  At least they
should be described.

(Though with x-debug-, it doesn't matter too much.)

> +
> +##
> +# @BlockGraphNode:
> +#

I think a description on at least what the name means for each type
would be useful; and that @id is generated just for this request and not
some significant value in the block layer.

> +# Since: 3.1
> +##
> +{ 'struct': 'BlockGraphNode',
> +  'data': { 'id': 'uint64', 'type': 'BlockGraphNodeType', 'name': 'str' } }
> +
> +##
> +# @BlockPermission:
> +#
> +# Enum of base block permissions.
> +#
> +# @consistent-read: A user that has the "permission" of consistent reads is
> +#                   guaranteed that their view of the contents of the block
> +#                   device is complete and self-consistent, representing the
> +#                   contents of a disk at a specific point.
> +#                   For most block devices (including their backing files) 
> this
> +#                   is true, but the property cannot be maintained in a few
> +#                   situations like for intermediate nodes of a commit block
> +#                   job.
> +#
> +# @write: This permission is required to change the visible disk contents.
> +#
> +# @write-unchanged: This permission (which is weaker than BLK_PERM_WRITE) is
> +#                   both enough and required for writes to the block node 
> when
> +#                   the caller promises that the visible disk content doesn't
> +#                   change.
> +#                   As the BLK_PERM_WRITE permission is strictly stronger,
> +#                   either is sufficient to perform an unchanging write.
> +#
> +# @resize: This permission is required to change the size of a block node.
> +#
> +# @graph-mod: This permission is required to change the node that this
> +#             BdrvChild points to.
> +#
> +# Since: 3.1
> +##
> +  { 'enum': 'BlockPermission',
> +    'data': [ 'consistent-read', 'write', 'write-unchanged', 'resize',
> +              'graph-mod' ] }
> +##
> +# @BlockGraphEdge:
> +#
> +# Block Graph edge description for x-debug-query-block-graph.
> +#
> +# @parent: parent id
> +#
> +# @child: child id
> +#
> +# @name: name of the relation (examples are 'file' and 'backing')
> +#
> +# @perm: granted permissions for the parent operating on the child
> +#
> +# @shared-perm: permissions that can still be granted to other users of the
> +#               child while it is still attached this parent

s/attached this/attached to this/

> +#
> +# Since: 3.1
> +##
> +{ 'struct': 'BlockGraphEdge',
> +  'data': { 'parent': 'uint64', 'child': 'uint64',
> +            'name': 'str', 'perm': [ 'BlockPermission' ],
> +            'shared-perm': [ 'BlockPermission' ] } }
> +
> +##
> +# @BlockGraph:
> +#
> +# Block Graph - list of nodes and list of edges.
> +#
> +# Since: 3.1
> +##
> +{ 'struct': 'BlockGraph',
> +  'data': { 'nodes': ['BlockGraphNode'], 'edges': ['BlockGraphEdge'] } }
> +
> +##
> +# @x-debug-query-block-graph:
> +#
> +# Get the block graph.
> +#
> +# Since: 3.1
> +##
> +{ 'command': 'x-debug-query-block-graph', 'returns': 'BlockGraph' }
> +
>  ##
>  # @drive-mirror:
>  #

[...]

> diff --git a/block.c b/block.c
> index 6161dbe3eb..588f5a2648 100644
> --- a/block.c
> +++ b/block.c
> @@ -4003,6 +4003,135 @@ BlockDeviceInfoList *bdrv_named_nodes_list(Error 
> **errp)
>      return list;
>  }
>  
> +#define QAPI_LIST_ADD(list, element) do { \
> +    typeof(list) _tmp = g_new(typeof(*(list)), 1); \
> +    _tmp->value = (element); \
> +    _tmp->next = (list); \
> +    list = _tmp; \

Interesting, how yoo put (list) as an rvalue in parentheses, but don't
do so when it's an lvalue. :-)

I can follow your line of thinking, but it should probably always be in
parentheses.

> +} while (0)
> +
> +typedef struct BlockGraphConstructor {
> +    BlockGraph *graph;
> +    GHashTable *hash;

Maybe...  "graph_nodes"?  "hash" seems awfully unspecific.

> +} BlockGraphConstructor;
> +
> +static BlockGraphConstructor *graph_new(void)

I think this is a very broad name.  Maybe prefix all of the function
names with debug_?  (Or dbg_, or dbg_blk, or something)

> +{
> +    BlockGraphConstructor *gr = g_new(BlockGraphConstructor, 1);
> +
> +    gr->graph = g_new0(BlockGraph, 1);
> +    gr->hash = g_hash_table_new(NULL, NULL);
> +
> +    return gr;
> +}
> +
> +static BlockGraph *graph_finalize(BlockGraphConstructor *gr)
> +{
> +    g_hash_table_destroy(gr->hash);
> +
> +    return gr->graph;

gr is leaked here.

> +}
> +
> +static uint64_t graph_node_num(BlockGraphConstructor *gr, void *node)
> +{
> +    uint64_t ret = (uint64_t)g_hash_table_lookup(gr->hash, node);

I'd prefer a cast to uintptr_t.  Otherwise the compiler may warn that
you cast a pointer to an integer of different size (with -m32).

Storing it in a uint64_t (with an implicit cast then) is OK, though.
But maybe you do want to store it in a uintptr_t.  The only reason not
to is because it's a uint64_t in the QAPI schema, but I think it'd be a
bit cleaner to work with uintptr_t internally and then emit it as
uint64_t (because that's definitely safe).  It's just a bit more honest
because this way it's clear that with -m32, we cannot even represent IDs
larger than 32 bit (which doesn't matter).

> +
> +    if (ret > 0) {

Just for style I'd prefer != over >.  That makes it more clear that this
is a NULL check and not a check for errors (represented as negative
integers, even though @ret is unsigned).

> +        return ret;
> +    }
> +
> +    ret = g_hash_table_size(gr->hash) + 1;

Maybe add a comment that you add 1 because 0 (NULL) is reserved for
non-entries?  (Yes, it's clear, or I wouldn't have figured it out, but
I'd still find it nice.)

> +    g_hash_table_insert(gr->hash, node, (void *)ret);

This definitely needs a uintptr_t cast or you'll get a warning for 32
bit pointers.

> +
> +    return ret;
> +}
> +
> +static void graph_add_node(BlockGraphConstructor *gr, void *node,
> +                           BlockGraphNodeType type, const char *name)
> +{
> +    BlockGraphNode *n;
> +
> +    n = g_new0(BlockGraphNode, 1);
> +
> +    n->id = graph_node_num(gr, node);
> +    n->type = type;
> +    n->name = g_strdup(name);
> +
> +    QAPI_LIST_ADD(gr->graph->nodes, n);
> +}
> +
> +static void graph_add_edge(BlockGraphConstructor *gr, void *parent,
> +                           const BdrvChild *child)
> +{
> +    typedef struct {
> +        unsigned int flag;
> +        BlockPermission num;
> +    } PermissionMap;
> +
> +    static PermissionMap permissions[] = {

You can add a const if you like.

> +        { BLK_PERM_CONSISTENT_READ, BLOCK_PERMISSION_CONSISTENT_READ },
> +        { BLK_PERM_WRITE,           BLOCK_PERMISSION_WRITE },
> +        { BLK_PERM_WRITE_UNCHANGED, BLOCK_PERMISSION_WRITE_UNCHANGED },
> +        { BLK_PERM_RESIZE,          BLOCK_PERMISSION_RESIZE },
> +        { BLK_PERM_GRAPH_MOD,       BLOCK_PERMISSION_GRAPH_MOD },
> +        { 0, 0 }
> +    };

You could add a static assertion that "1 << (sizeof(permissions) /
sizeof(permissions[0]) - 1) == BLK_PERM_ALL + 1" to ensure that whenever
we add a permission, we don't forget to update this list.  But then
again, I don't think we're going to add a permission any time soon...

> +    PermissionMap *p;
> +    BlockGraphEdge *edge;
> +
> +    edge = g_new0(BlockGraphEdge, 1);
> +
> +    edge->parent = graph_node_num(gr, parent);
> +    edge->child = graph_node_num(gr, child->bs);
> +    edge->name = g_strdup(child->name);
> +
> +    for (p = permissions; p->flag; p++) {
> +        if (p->flag & child->perm) {
> +            QAPI_LIST_ADD(edge->perm, p->num);
> +        }
> +        if (p->flag & child->shared_perm) {
> +            QAPI_LIST_ADD(edge->shared_perm, p->num);
> +        }
> +    }
> +
> +    QAPI_LIST_ADD(gr->graph->edges, edge);
> +}
> +
> +
> +BlockGraph *bdrv_get_block_graph(Error **errp)
> +{
> +    BlockBackend *blk;
> +    BlockJob *job;
> +    BlockDriverState *bs;
> +    BdrvChild *child;
> +    BlockGraphConstructor *gr = graph_new();
> +
> +    for (blk = blk_all_next(NULL); blk; blk = blk_all_next(blk)) {
> +        graph_add_node(gr, blk, BLOCK_GRAPH_NODE_TYPE_BLK, blk_name(blk));

Hm, I think blk_name() is empty for most backends.
blk_root_get_parent_desc() then falls back to blk_get_attached_dev_id().
 I think we should do the same here.

Max

> +        if (blk_root(blk)) {
> +            graph_add_edge(gr, blk, blk_root(blk));
> +        }
> +    }
> +
> +    for (job = block_job_next(NULL); job; job = block_job_next(job)) {
> +        GSList *el;
> +
> +        graph_add_node(gr, job, BLOCK_GRAPH_NODE_TYPE_JOB, job->job.id);
> +        for (el = job->nodes; el; el = el->next) {
> +            graph_add_edge(gr, job, (BdrvChild *)el->data);
> +        }
> +    }
> +
> +    QTAILQ_FOREACH(bs, &graph_bdrv_states, node_list) {
> +        graph_add_node(gr, bs, BLOCK_GRAPH_NODE_TYPE_BDS, bs->node_name);
> +        QLIST_FOREACH(child, &bs->children, next) {
> +            graph_add_edge(gr, bs, child);
> +        }
> +    }
> +
> +    return graph_finalize(gr);
> +}
> +
>  BlockDriverState *bdrv_lookup_bs(const char *device,
>                                   const char *node_name,
>                                   Error **errp)

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