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>
Hanna Czenczek committed
Mar 9, 2026 at 16:08 UTC
277fc1ce184a7d4df6790a3deeffd4025e3ad7d0
7 files changed
+113
-11
block/export/export.c
+44
-4
@@ -76,16 +76,26 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
76
{
77
bool fixed_iothread = export->has_fixed_iothread && export->fixed_iothread;
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bool allow_inactive = export->has_allow_inactive && export->allow_inactive;
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+ bool multithread = export->iothread &&
80
+ export->iothread->type == QTYPE_QLIST;
81
const BlockExportDriver *drv;
82
BlockExport *exp = NULL;
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BlockDriverState *bs;
84
BlockBackend *blk = NULL;
85
AioContext *ctx;
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+ AioContext **multithread_ctxs = NULL;
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+ size_t multithread_count = 0;
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uint64_t perm;
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int ret;
90
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GLOBAL_STATE_CODE();
92
93
+ if (fixed_iothread && multithread) {
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+ error_setg(errp,
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+ "Cannot use fixed-iothread for a multi-threaded export");
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+ return NULL;
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+ }
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+
99
if (!id_wellformed(export->id)) {
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error_setg(errp, "Invalid block export id");
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return NULL;
@@ -116,14 +126,16 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
126
127
ctx = bdrv_get_aio_context(bs);
128
119
- if (export->iothread) {
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+ /* Move the BDS to the target I/O thread, if it is a single one */
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+ if (export->iothread && !multithread) {
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+ const char *iothread_id = export->iothread->u.single;
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IOThread *iothread;
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AioContext *new_ctx;
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Error **set_context_errp;
135
124
- iothread = iothread_by_id(export->iothread);
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+ iothread = iothread_by_id(iothread_id);
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if (!iothread) {
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- error_setg(errp, "iothread \"%s\" not found", export->iothread);
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+ error_setg(errp, "iothread \"%s\" not found", iothread_id);
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goto fail;
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}
141
@@ -137,6 +149,32 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
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} else if (fixed_iothread) {
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goto fail;
151
}
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+ } else if (multithread) {
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+ strList *iothread_list = export->iothread->u.multi;
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+ size_t i;
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+
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+ multithread_count = 0;
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+ for (strList *e = iothread_list; e; e = e->next) {
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+ multithread_count++;
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+ }
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+
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+ if (multithread_count == 0) {
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+ error_setg(errp, "The set of I/O threads must not be empty");
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+ return NULL;
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+ }
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+
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+ multithread_ctxs = g_new(AioContext *, multithread_count);
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+ i = 0;
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+ for (strList *e = iothread_list; e; e = e->next) {
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+ IOThread *iothread = iothread_by_id(e->value);
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+
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+ if (!iothread) {
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+ error_setg(errp, "iothread \"%s\" not found", e->value);
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+ goto fail;
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+ }
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+ multithread_ctxs[i++] = iothread_get_aio_context(iothread);
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+ }
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+ assert(i == multithread_count);
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}
179
180
bdrv_graph_rdlock_main_loop();
@@ -195,7 +233,7 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
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.blk = blk,
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};
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198
- ret = drv->create(exp, export, errp);
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+ ret = drv->create(exp, export, multithread_ctxs, multithread_count, errp);
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if (ret < 0) {
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goto fail;
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}
@@ -203,6 +241,7 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
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assert(exp->blk != NULL);
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QLIST_INSERT_HEAD(&block_exports, exp, next);
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+ g_free(multithread_ctxs);
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return exp;
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247
fail:
@@ -214,6 +253,7 @@ fail:
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g_free(exp->id);
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g_free(exp);
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}
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+ g_free(multithread_ctxs);
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return NULL;
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}
259
block/export/fuse.c
+7
@@ -259,6 +259,8 @@ static const BlockDevOps fuse_export_blk_dev_ops = {
259
260
static int fuse_export_create(BlockExport *blk_exp,
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BlockExportOptions *blk_exp_args,
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+ AioContext *const *multithread,
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+ size_t mt_count,
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Error **errp)
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{
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ERRP_GUARD(); /* ensure clean-up even with error_fatal */
@@ -268,6 +270,11 @@ static int fuse_export_create(BlockExport *blk_exp,
270
271
assert(blk_exp_args->type == BLOCK_EXPORT_TYPE_FUSE);
272
273
+ if (multithread) {
274
+ error_setg(errp, "FUSE export does not support multi-threading");
275
+ return -EINVAL;
276
+ }
277
+
278
/* For growable and writable exports, take the RESIZE permission */
279
if (args->growable || blk_exp_args->writable) {
280
uint64_t blk_perm, blk_shared_perm;
block/export/vduse-blk.c
+7
@@ -267,6 +267,7 @@ static const BlockDevOps vduse_block_ops = {
267
};
268
269
static int vduse_blk_exp_create(BlockExport *exp, BlockExportOptions *opts,
270
+ AioContext *const *multithread, size_t mt_count,
271
Error **errp)
272
{
273
VduseBlkExport *vblk_exp = container_of(exp, VduseBlkExport, export);
@@ -302,6 +303,12 @@ static int vduse_blk_exp_create(BlockExport *exp, BlockExportOptions *opts,
303
return -EINVAL;
304
}
305
}
306
+
307
+ if (multithread) {
308
+ error_setg(errp, "vduse-blk export does not support multi-threading");
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+ return -EINVAL;
310
+ }
311
+
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vblk_exp->num_queues = num_queues;
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vblk_exp->handler.blk = exp->blk;
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vblk_exp->handler.serial = g_strdup(vblk_opts->serial ?: "");
block/export/vhost-user-blk-server.c
+8
@@ -316,6 +316,7 @@ static const BlockDevOps vu_blk_dev_ops = {
316
};
317
318
static int vu_blk_exp_create(BlockExport *exp, BlockExportOptions *opts,
319
+ AioContext *const *multithread, size_t mt_count,
320
Error **errp)
321
{
322
VuBlkExport *vexp = container_of(exp, VuBlkExport, export);
@@ -341,6 +342,13 @@ static int vu_blk_exp_create(BlockExport *exp, BlockExportOptions *opts,
342
error_setg(errp, "num-queues must be greater than 0");
343
return -EINVAL;
344
}
345
+
346
+ if (multithread) {
347
+ error_setg(errp,
348
+ "vhost-user-blk export does not support multi-threading");
349
+ return -EINVAL;
350
+ }
351
+
352
vexp->handler.blk = exp->blk;
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vexp->handler.serial = g_strdup("vhost_user_blk");
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vexp->handler.logical_block_size = logical_block_size;
include/block/export.h
+10
-2
@@ -32,8 +32,16 @@ typedef struct BlockExportDriver {
32
/* True if the export type supports running on an inactive node */
33
bool supports_inactive;
34
35
- /* Creates and starts a new block export */
36
- int (*create)(BlockExport *, BlockExportOptions *, Error **);
35
+ /*
36
+ * Creates and starts a new block export.
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+ *
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+ * If the user passed a set of I/O threads for multi-threading, @multithread
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+ * is a list of the @multithread_count corresponding contexts (freed by the
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+ * caller). Note that @exp->ctx has no relation to that list.
41
+ */
42
+ int (*create)(BlockExport *exp, BlockExportOptions *opts,
43
+ AioContext *const *multithread, size_t multithread_count,
44
+ Error **errp);
45
46
/*
47
* Frees a removed block export. This function is only called after all
nbd/server.c
+6
@@ -1795,6 +1795,7 @@ static const BlockDevOps nbd_block_ops = {
1795
};
1796
1797
static int nbd_export_create(BlockExport *blk_exp, BlockExportOptions *exp_args,
1798
+ AioContext *const *multithread, size_t mt_count,
1799
Error **errp)
1800
{
1801
NBDExport *exp = container_of(blk_exp, NBDExport, common);
@@ -1831,6 +1832,11 @@ static int nbd_export_create(BlockExport *blk_exp, BlockExportOptions *exp_args,
1832
return -EEXIST;
1833
}
1834
1835
+ if (multithread) {
1836
+ error_setg(errp, "NBD export does not support multi-threading");
1837
+ return -EINVAL;
1838
+ }
1839
+
1840
size = blk_getlength(blk);
1841
if (size < 0) {
1842
error_setg_errno(errp, -size,
qapi/block-export.json
+31
-5
@@ -363,14 +363,16 @@
363
# to the export before completion is signalled. (since: 5.2;
364
# default: false)
365
#
366
-# @iothread: The name of the iothread object where the export will
367
-# run. The default is to use the thread currently associated with
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-# the block node. (since: 5.2)
366
+# @iothread: The name(s) of one or more iothread object(s) where the
367
+# export will run. The default is to use the thread currently
368
+# associated with the block node. (since: 5.2; multi-threading
369
+# since 10.1)
370
#
371
# @fixed-iothread: True prevents the block node from being moved to
372
# another thread while the export is active. If true and
373
# @iothread is given, export creation fails if the block node
373
-# cannot be moved to the iothread. The default is false.
374
+# cannot be moved to the iothread. Must not be true when giving
375
+# multiple iothreads for @iothread. The default is false.
376
# (since: 5.2)
377
#
378
# @allow-inactive: If true, the export allows the exported node to be
@@ -387,7 +389,7 @@
389
'base': { 'type': 'BlockExportType',
390
'id': 'str',
391
'*fixed-iothread': 'bool',
390
- '*iothread': 'str',
392
+ '*iothread': 'BlockExportIothreads',
393
'node-name': 'str',
394
'*writable': 'bool',
395
'*writethrough': 'bool',
@@ -403,6 +405,30 @@
405
'if': 'CONFIG_VDUSE_BLK_EXPORT' }
406
} }
407
408
+##
409
+# @BlockExportIothreads:
410
+#
411
+# Specify a single or multiple I/O threads in which to run a block
412
+# export's I/O.
413
+#
414
+# @single: Run the export's I/O in the given single I/O thread.
415
+#
416
+# @multi: Use multi-threading across the given set of I/O threads,
417
+# which must not be empty. Note: Passing a single I/O thread via
418
+# this variant is still treated as multi-threading, which is
419
+# different from using the @single variant. In particular, even
420
+# if there only is a single I/O thread in the set, export types
421
+# that do not support multi-threading will generally reject this
422
+# variant, and BlockExportOptions.fixed-iothread is always
423
+# incompatible with it.
424
+#
425
+# Since: 10.1
426
+##
427
+{ 'alternate': 'BlockExportIothreads',
428
+ 'data': {
429
+ 'single': 'str',
430
+ 'multi': ['str'] } }
431
+
432
##
433
# @block-export-add:
434
#