Add a string tag property to HostMemoryBackend so that backends can be
identified by a user-assigned name at runtime. Expose the property through
QOM and add a host_memory_backend_find_by_tag() helper that walks the
object tree to locate a backend by its tag.
This is used by the following CXL patches to resolve a named backend for a
pending DC extent and attach it to the accepted extent when the host
acknowledges the add request.
Signed-off-by: Alireza Sanaee <alireza.sanaee@huawei.com>
---
backends/hostmem.c | 72 ++++++++++++++++++++++++++++++++++++++++
include/system/hostmem.h | 2 ++
qapi/qom.json | 3 ++
tests/qtest/qom-test.c | 8 +++--
4 files changed, 83 insertions(+), 2 deletions(-)
diff --git a/backends/hostmem.c b/backends/hostmem.c
index 15d4365b69..270cae9569 100644
--- a/backends/hostmem.c
+++ b/backends/hostmem.c
@@ -47,6 +47,50 @@ host_memory_backend_get_name(HostMemoryBackend *backend)
return object_get_canonical_path(OBJECT(backend));
}
+typedef struct HostMemoryBackendTagSearch {
+ const char *tag;
+ HostMemoryBackend *backend;
+} HostMemoryBackendTagSearch;
+
+static int host_memory_backend_find_by_tag_cb(Object *obj, void *opaque)
+{
+ HostMemoryBackendTagSearch *ctx = opaque;
+ HostMemoryBackend *backend;
+
+ if (!object_dynamic_cast(obj, TYPE_MEMORY_BACKEND)) {
+ return 0;
+ }
+
+ backend = MEMORY_BACKEND(obj);
+ if (!backend->tag) {
+ return 0;
+ }
+
+ if (strcmp(backend->tag, ctx->tag) == 0) {
+ ctx->backend = backend;
+ return 1;
+ }
+
+ return 0;
+}
+
+HostMemoryBackend *host_memory_backend_find_by_tag(const char *tag)
+{
+ HostMemoryBackendTagSearch ctx = {
+ .tag = tag,
+ .backend = NULL,
+ };
+
+ if (!tag) {
+ return NULL;
+ }
+
+ object_child_foreach_recursive(object_get_objects_root(),
+ host_memory_backend_find_by_tag_cb, &ctx);
+
+ return ctx.backend;
+}
+
static void
host_memory_backend_get_size(Object *obj, Visitor *v, const char *name,
void *opaque, Error **errp)
@@ -460,6 +504,22 @@ static void host_memory_backend_set_share(Object *o, bool value, Error **errp)
backend->share = value;
}
+static void
+host_memory_backend_set_tag(Object *obj, const char *value, Error **errp)
+{
+ HostMemoryBackend *backend = MEMORY_BACKEND(obj);
+
+ g_free(backend->tag);
+ backend->tag = g_strdup(value);
+}
+
+static char *host_memory_backend_get_tag(Object *obj, Error **errp)
+{
+ HostMemoryBackend *backend = MEMORY_BACKEND(obj);
+
+ return g_strdup(backend->tag);
+}
+
#ifdef CONFIG_LINUX
static bool host_memory_backend_get_reserve(Object *o, Error **errp)
{
@@ -501,6 +561,13 @@ host_memory_backend_set_use_canonical_path(Object *obj, bool value,
backend->use_canonical_path = value;
}
+static void host_memory_backend_finalize(Object *obj)
+{
+ HostMemoryBackend *backend = MEMORY_BACKEND(obj);
+
+ g_free(backend->tag);
+}
+
static void
host_memory_backend_class_init(ObjectClass *oc, const void *data)
{
@@ -557,6 +624,10 @@ host_memory_backend_class_init(ObjectClass *oc, const void *data)
host_memory_backend_get_share, host_memory_backend_set_share);
object_class_property_set_description(oc, "share",
"Mark the memory as private to QEMU or shared");
+ object_class_property_add_str(oc, "tag",
+ host_memory_backend_get_tag, host_memory_backend_set_tag);
+ object_class_property_set_description(oc, "tag",
+ "A user-defined tag to identify this memory backend");
#ifdef CONFIG_LINUX
object_class_property_add_bool(oc, "reserve",
host_memory_backend_get_reserve, host_memory_backend_set_reserve);
@@ -586,6 +657,7 @@ static const TypeInfo host_memory_backend_info = {
.class_init = host_memory_backend_class_init,
.instance_size = sizeof(HostMemoryBackend),
.instance_init = host_memory_backend_init,
+ .instance_finalize = host_memory_backend_finalize,
.instance_post_init = host_memory_backend_post_init,
.interfaces = (const InterfaceInfo[]) {
{ TYPE_USER_CREATABLE },
diff --git a/include/system/hostmem.h b/include/system/hostmem.h
index 88fa791ac7..a02e173d48 100644
--- a/include/system/hostmem.h
+++ b/include/system/hostmem.h
@@ -81,12 +81,14 @@ struct HostMemoryBackend {
ThreadContext *prealloc_context;
DECLARE_BITMAP(host_nodes, MAX_NODES + 1);
HostMemPolicy policy;
+ char *tag;
MemoryRegion mr;
};
bool host_memory_backend_mr_inited(HostMemoryBackend *backend);
MemoryRegion *host_memory_backend_get_memory(HostMemoryBackend *backend);
+HostMemoryBackend *host_memory_backend_find_by_tag(const char *tag);
void host_memory_backend_set_mapped(HostMemoryBackend *backend, bool mapped);
bool host_memory_backend_is_mapped(HostMemoryBackend *backend);
diff --git a/qapi/qom.json b/qapi/qom.json
index c653248f85..e6feeb324a 100644
--- a/qapi/qom.json
+++ b/qapi/qom.json
@@ -658,6 +658,8 @@
# @reserve: if true, reserve swap space (or huge pages) if applicable
# (default: true) (since 6.1)
#
+# @tag: user-defined memory backend tag (since 11.1)
+#
# @size: size of the memory region in bytes
#
# @x-use-canonical-path-for-ramblock-id: if true, the canonical path
@@ -683,6 +685,7 @@
'*prealloc-context': 'str',
'*share': 'bool',
'*reserve': 'bool',
+ '*tag': 'str',
'size': 'size',
'*x-use-canonical-path-for-ramblock-id': 'bool' } }
diff --git a/tests/qtest/qom-test.c b/tests/qtest/qom-test.c
index 6421f2d9d9..3109b47cca 100644
--- a/tests/qtest/qom-test.c
+++ b/tests/qtest/qom-test.c
@@ -17,6 +17,7 @@
#define RAM_NAME "node0"
#define RAM_SIZE 65536
+#define RAM_TAG "ramtag0"
static int verbosity_level;
@@ -59,6 +60,8 @@ static void test_list_get_value(QTestState *qts)
} else if (!strcmp(prop_name, "size")) {
g_assert_cmpint(qdict_get_int(prop, "value"), ==, RAM_SIZE);
+ } else if (!strcmp(prop_name, "tag")) {
+ g_assert_cmpstr(qdict_get_str(prop, "value"), ==, RAM_TAG);
}
}
}
@@ -195,8 +198,9 @@ static void test_machine(gconstpointer data)
QTestState *qts;
g_autoptr(QList) paths = qlist_new();
- qts = qtest_initf("-machine %s -object memory-backend-ram,id=%s,size=%d",
- machine, RAM_NAME, RAM_SIZE);
+ qts = qtest_initf("-machine %s -object memory-backend-ram,id=%s,size=%d"
+ ",tag=%s",
+ machine, RAM_NAME, RAM_SIZE, RAM_TAG);
if (g_test_slow()) {
/* Make sure we can get the machine class properties: */
--
2.50.1 (Apple Git-155)Alireza Sanaee <alireza.sanaee@huawei.com> writes: > Add a string tag property to HostMemoryBackend so that backends can be > identified by a user-assigned name at runtime. Expose the property through > QOM and add a host_memory_backend_find_by_tag() helper that walks the > object tree to locate a backend by its tag. Why can't you use the QOM ID? $ qemu-system-x86_64 -S -display none -monitor stdio -object memory-backend-ram,id=fred,size=1M QEMU 10.2.90 monitor - type 'help' for more information (qemu) info qom-tree /objects/fred /fred (memory-backend-ram) /fred[0] (memory-region)
On Thu, 26 Mar 2026 11:42:17 +0100 Markus Armbruster <armbru@redhat.com> wrote: Hi Markus, > Alireza Sanaee <alireza.sanaee@huawei.com> writes: > > > Add a string tag property to HostMemoryBackend so that backends can be > > identified by a user-assigned name at runtime. Expose the property through > > QOM and add a host_memory_backend_find_by_tag() helper that walks the > > object tree to locate a backend by its tag. > > Why can't you use the QOM ID? > > $ qemu-system-x86_64 -S -display none -monitor stdio -object memory-backend-ram,id=fred,size=1M > QEMU 10.2.90 monitor - type 'help' for more information > (qemu) info qom-tree /objects/fred > /fred (memory-backend-ram) > /fred[0] (memory-region) > > Fair question. The ID would have been a solution if it had accepted GUIDs. CXL spec requires GUIDs for extents as identification, hence new tag property. alireza@blue-ocean:~/Downloads$ qemu-system-x86_64 -s -display none -monitor std io -object memory-backend-ram,id=02bb2436-e87d-4aa1-b2be-aaffdd76bb79,size=1M qemu-system-x86_64: -object memory-backend-ram,id=02bb2436-e87d-4aa1-b2be-aaffdd 76bb79,size=1M: Parameter 'id' expects an identifier Identifiers consist of letters, digits, '-', '.', '_', starting with a letter. Thanks, Ali
Alireza Sanaee <alireza.sanaee@huawei.com> writes: > On Thu, 26 Mar 2026 11:42:17 +0100 > Markus Armbruster <armbru@redhat.com> wrote: > > Hi Markus, > >> Alireza Sanaee <alireza.sanaee@huawei.com> writes: >> >> > Add a string tag property to HostMemoryBackend so that backends can be >> > identified by a user-assigned name at runtime. Expose the property through >> > QOM and add a host_memory_backend_find_by_tag() helper that walks the >> > object tree to locate a backend by its tag. >> >> Why can't you use the QOM ID? >> >> $ qemu-system-x86_64 -S -display none -monitor stdio -object memory-backend-ram,id=fred,size=1M >> QEMU 10.2.90 monitor - type 'help' for more information >> (qemu) info qom-tree /objects/fred >> /fred (memory-backend-ram) >> /fred[0] (memory-region) >> >> > > Fair question. The ID would have been a solution if it had accepted GUIDs. CXL spec > requires GUIDs for extents as identification, hence new tag property. Your commit message should state that this needs to be a GUID and why. > alireza@blue-ocean:~/Downloads$ qemu-system-x86_64 -s -display none -monitor std > io -object memory-backend-ram,id=02bb2436-e87d-4aa1-b2be-aaffdd76bb79,size=1M > qemu-system-x86_64: -object memory-backend-ram,id=02bb2436-e87d-4aa1-b2be-aaffdd > 76bb79,size=1M: Parameter 'id' expects an identifier > Identifiers consist of letters, digits, '-', '.', '_', starting with a letter. > > Thanks, > Ali
On Thu, 26 Mar 2026 14:01:06 +0100 Markus Armbruster <armbru@redhat.com> wrote: Hi Markus, Sure, absolutely. > Alireza Sanaee <alireza.sanaee@huawei.com> writes: > > > On Thu, 26 Mar 2026 11:42:17 +0100 > > Markus Armbruster <armbru@redhat.com> wrote: > > > > Hi Markus, > > > >> Alireza Sanaee <alireza.sanaee@huawei.com> writes: > >> > >> > Add a string tag property to HostMemoryBackend so that backends can be > >> > identified by a user-assigned name at runtime. Expose the property through > >> > QOM and add a host_memory_backend_find_by_tag() helper that walks the > >> > object tree to locate a backend by its tag. > >> > >> Why can't you use the QOM ID? > >> > >> $ qemu-system-x86_64 -S -display none -monitor stdio -object memory-backend-ram,id=fred,size=1M > >> QEMU 10.2.90 monitor - type 'help' for more information > >> (qemu) info qom-tree /objects/fred > >> /fred (memory-backend-ram) > >> /fred[0] (memory-region) > >> > >> > > > > Fair question. The ID would have been a solution if it had accepted GUIDs. CXL spec > > requires GUIDs for extents as identification, hence new tag property. > > Your commit message should state that this needs to be a GUID and why. > > > alireza@blue-ocean:~/Downloads$ qemu-system-x86_64 -s -display none -monitor std > > io -object memory-backend-ram,id=02bb2436-e87d-4aa1-b2be-aaffdd76bb79,size=1M > > qemu-system-x86_64: -object memory-backend-ram,id=02bb2436-e87d-4aa1-b2be-aaffdd > > 76bb79,size=1M: Parameter 'id' expects an identifier > > Identifiers consist of letters, digits, '-', '.', '_', starting with a letter. > > > > Thanks, > > Ali > >
© 2016 - 2026 Red Hat, Inc.