block/export: Add multi-threading interface

Make BlockExportType.iothread an alternate between a single-thread
variant 'str' and a multi-threading variant '[str]'.

In contrast to the single-thread setting, the multi-threading setting
will not change the BDS's context (and so is incompatible with the
fixed-iothread setting), but instead just pass a list to the export
driver, with which it can do whatever it wants.

Currently no export driver supports multi-threading, so they all return
an error when receiving such a list.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-ID: <20260309150856.26800-21-hreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/export/export.c

@@ -76,16 +76,26 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
 {
     bool fixed_iothread = export->has_fixed_iothread && export->fixed_iothread;
     bool allow_inactive = export->has_allow_inactive && export->allow_inactive;
+    bool multithread = export->iothread &&
+        export->iothread->type == QTYPE_QLIST;
     const BlockExportDriver *drv;
     BlockExport *exp = NULL;
     BlockDriverState *bs;
     BlockBackend *blk = NULL;
     AioContext *ctx;
+    AioContext **multithread_ctxs = NULL;
+    size_t multithread_count = 0;
     uint64_t perm;
     int ret;
 
     GLOBAL_STATE_CODE();
 
+    if (fixed_iothread && multithread) {
+        error_setg(errp,
+                   "Cannot use fixed-iothread for a multi-threaded export");
+        return NULL;
+    }
+
     if (!id_wellformed(export->id)) {
         error_setg(errp, "Invalid block export id");
         return NULL;
@@ -116,14 +126,16 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
 
     ctx = bdrv_get_aio_context(bs);
 
-    if (export->iothread) {
+    /* Move the BDS to the target I/O thread, if it is a single one */
+    if (export->iothread && !multithread) {
+        const char *iothread_id = export->iothread->u.single;
         IOThread *iothread;
         AioContext *new_ctx;
         Error **set_context_errp;
 
-        iothread = iothread_by_id(export->iothread);
+        iothread = iothread_by_id(iothread_id);
         if (!iothread) {
-            error_setg(errp, "iothread \"%s\" not found", export->iothread);
+            error_setg(errp, "iothread \"%s\" not found", iothread_id);
             goto fail;
         }
 
@@ -137,6 +149,32 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
         } else if (fixed_iothread) {
             goto fail;
         }
+    } else if (multithread) {
+        strList *iothread_list = export->iothread->u.multi;
+        size_t i;
+
+        multithread_count = 0;
+        for (strList *e = iothread_list; e; e = e->next) {
+            multithread_count++;
+        }
+
+        if (multithread_count == 0) {
+            error_setg(errp, "The set of I/O threads must not be empty");
+            return NULL;
+        }
+
+        multithread_ctxs = g_new(AioContext *, multithread_count);
+        i = 0;
+        for (strList *e = iothread_list; e; e = e->next) {
+            IOThread *iothread = iothread_by_id(e->value);
+
+            if (!iothread) {
+                error_setg(errp, "iothread \"%s\" not found", e->value);
+                goto fail;
+            }
+            multithread_ctxs[i++] = iothread_get_aio_context(iothread);
+        }
+        assert(i == multithread_count);
     }
 
     bdrv_graph_rdlock_main_loop();
@@ -195,7 +233,7 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
         .blk        = blk,
     };
 
-    ret = drv->create(exp, export, errp);
+    ret = drv->create(exp, export, multithread_ctxs, multithread_count, errp);
     if (ret < 0) {
         goto fail;
     }
@@ -203,6 +241,7 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
     assert(exp->blk != NULL);
 
     QLIST_INSERT_HEAD(&block_exports, exp, next);
+    g_free(multithread_ctxs);
     return exp;
 
 fail:
@@ -214,6 +253,7 @@ fail:
         g_free(exp->id);
         g_free(exp);
     }
+    g_free(multithread_ctxs);
     return NULL;
 }
 

block/export/fuse.c

@@ -259,6 +259,8 @@ static const BlockDevOps fuse_export_blk_dev_ops = {
 
 static int fuse_export_create(BlockExport *blk_exp,
                               BlockExportOptions *blk_exp_args,
+                              AioContext *const *multithread,
+                              size_t mt_count,
                               Error **errp)
 {
     ERRP_GUARD(); /* ensure clean-up even with error_fatal */
@@ -268,6 +270,11 @@ static int fuse_export_create(BlockExport *blk_exp,
 
     assert(blk_exp_args->type == BLOCK_EXPORT_TYPE_FUSE);
 
+    if (multithread) {
+        error_setg(errp, "FUSE export does not support multi-threading");
+        return -EINVAL;
+    }
+
     /* For growable and writable exports, take the RESIZE permission */
     if (args->growable || blk_exp_args->writable) {
         uint64_t blk_perm, blk_shared_perm;

block/export/vduse-blk.c

@@ -267,6 +267,7 @@ static const BlockDevOps vduse_block_ops = {
 };
 
 static int vduse_blk_exp_create(BlockExport *exp, BlockExportOptions *opts,
+                                AioContext *const *multithread, size_t mt_count,
                                 Error **errp)
 {
     VduseBlkExport *vblk_exp = container_of(exp, VduseBlkExport, export);
@@ -302,6 +303,12 @@ static int vduse_blk_exp_create(BlockExport *exp, BlockExportOptions *opts,
             return -EINVAL;
         }
     }
+
+    if (multithread) {
+        error_setg(errp, "vduse-blk export does not support multi-threading");
+        return -EINVAL;
+    }
+
     vblk_exp->num_queues = num_queues;
     vblk_exp->handler.blk = exp->blk;
     vblk_exp->handler.serial = g_strdup(vblk_opts->serial ?: "");

block/export/vhost-user-blk-server.c

@@ -316,6 +316,7 @@ static const BlockDevOps vu_blk_dev_ops = {
 };
 
 static int vu_blk_exp_create(BlockExport *exp, BlockExportOptions *opts,
+                             AioContext *const *multithread, size_t mt_count,
                              Error **errp)
 {
     VuBlkExport *vexp = container_of(exp, VuBlkExport, export);
@@ -341,6 +342,13 @@ static int vu_blk_exp_create(BlockExport *exp, BlockExportOptions *opts,
         error_setg(errp, "num-queues must be greater than 0");
         return -EINVAL;
     }
+
+    if (multithread) {
+        error_setg(errp,
+                   "vhost-user-blk export does not support multi-threading");
+        return -EINVAL;
+    }
+
     vexp->handler.blk = exp->blk;
     vexp->handler.serial = g_strdup("vhost_user_blk");
     vexp->handler.logical_block_size = logical_block_size;

include/block/export.h

@@ -32,8 +32,16 @@ typedef struct BlockExportDriver {
     /* True if the export type supports running on an inactive node */
     bool supports_inactive;
 
-    /* Creates and starts a new block export */
-    int (*create)(BlockExport *, BlockExportOptions *, Error **);
+    /*
+     * Creates and starts a new block export.
+     *
+     * If the user passed a set of I/O threads for multi-threading, @multithread
+     * is a list of the @multithread_count corresponding contexts (freed by the
+     * caller).  Note that @exp->ctx has no relation to that list.
+     */
+    int (*create)(BlockExport *exp, BlockExportOptions *opts,
+                  AioContext *const *multithread, size_t multithread_count,
+                  Error **errp);
 
     /*
      * Frees a removed block export. This function is only called after all

nbd/server.c

@@ -1795,6 +1795,7 @@ static const BlockDevOps nbd_block_ops = {
 };
 
 static int nbd_export_create(BlockExport *blk_exp, BlockExportOptions *exp_args,
+                             AioContext *const *multithread, size_t mt_count,
                              Error **errp)
 {
     NBDExport *exp = container_of(blk_exp, NBDExport, common);
@@ -1831,6 +1832,11 @@ static int nbd_export_create(BlockExport *blk_exp, BlockExportOptions *exp_args,
         return -EEXIST;
     }
 
+    if (multithread) {
+        error_setg(errp, "NBD export does not support multi-threading");
+        return -EINVAL;
+    }
+
     size = blk_getlength(blk);
     if (size < 0) {
         error_setg_errno(errp, -size,

qapi/block-export.json

@@ -363,14 +363,16 @@
 #     to the export before completion is signalled.  (since: 5.2;
 #     default: false)
 #
-# @iothread: The name of the iothread object where the export will
-#     run.  The default is to use the thread currently associated with
-#     the block node.  (since: 5.2)
+# @iothread: The name(s) of one or more iothread object(s) where the
+#     export will run.  The default is to use the thread currently
+#     associated with the block node.  (since: 5.2; multi-threading
+#     since 10.1)
 #
 # @fixed-iothread: True prevents the block node from being moved to
 #     another thread while the export is active.  If true and
 #     @iothread is given, export creation fails if the block node
-#     cannot be moved to the iothread.  The default is false.
+#     cannot be moved to the iothread.  Must not be true when giving
+#     multiple iothreads for @iothread.  The default is false.
 #     (since: 5.2)
 #
 # @allow-inactive: If true, the export allows the exported node to be
@@ -387,7 +389,7 @@
   'base': { 'type': 'BlockExportType',
             'id': 'str',
             '*fixed-iothread': 'bool',
-            '*iothread': 'str',
+            '*iothread': 'BlockExportIothreads',
             'node-name': 'str',
             '*writable': 'bool',
             '*writethrough': 'bool',
@@ -403,6 +405,30 @@
                      'if': 'CONFIG_VDUSE_BLK_EXPORT' }
    } }
 
+##
+# @BlockExportIothreads:
+#
+# Specify a single or multiple I/O threads in which to run a block
+# export's I/O.
+#
+# @single: Run the export's I/O in the given single I/O thread.
+#
+# @multi: Use multi-threading across the given set of I/O threads,
+#     which must not be empty.  Note: Passing a single I/O thread via
+#     this variant is still treated as multi-threading, which is
+#     different from using the @single variant.  In particular, even
+#     if there only is a single I/O thread in the set, export types
+#     that do not support multi-threading will generally reject this
+#     variant, and BlockExportOptions.fixed-iothread is always
+#     incompatible with it.
+#
+# Since: 10.1
+##
+{ 'alternate': 'BlockExportIothreads',
+  'data': {
+      'single': 'str',
+      'multi': ['str'] } }
+
 ##
 # @block-export-add:
 #