@@ -98,8 +98,9 @@ Shared memory object
In order for the daemon to access the VirtIO queues to process the
requests it needs access to the guest's address space. This is
-achieved via the ``memory-backend-file`` or ``memory-backend-memfd``
-objects. A reference to a file-descriptor which can access this object
+achieved via the ``memory-backend-file``, ``memory-backend-memfd``, or
+``memory-backend-shm`` objects.
+A reference to a file-descriptor which can access this object
will be passed via the socket as part of the protocol negotiation.
Currently the shared memory object needs to match the size of the main
@@ -721,6 +721,21 @@
'*hugetlbsize': 'size',
'*seal': 'bool' } }
+##
+# @MemoryBackendShmProperties:
+#
+# Properties for memory-backend-shm objects.
+#
+# The @share boolean option is true by default with shm. Setting it to false
+# will cause a failure during allocation because it is not supported by this
+# backend.
+#
+# Since: 9.1
+##
+{ 'struct': 'MemoryBackendShmProperties',
+ 'base': 'MemoryBackendProperties',
+ 'data': { } }
+
##
# @MemoryBackendEpcProperties:
#
@@ -985,6 +1000,8 @@
{ 'name': 'memory-backend-memfd',
'if': 'CONFIG_LINUX' },
'memory-backend-ram',
+ { 'name': 'memory-backend-shm',
+ 'if': 'CONFIG_POSIX' },
'pef-guest',
{ 'name': 'pr-manager-helper',
'if': 'CONFIG_LINUX' },
@@ -1056,6 +1073,8 @@
'memory-backend-memfd': { 'type': 'MemoryBackendMemfdProperties',
'if': 'CONFIG_LINUX' },
'memory-backend-ram': 'MemoryBackendProperties',
+ 'memory-backend-shm': { 'type': 'MemoryBackendShmProperties',
+ 'if': 'CONFIG_POSIX' },
'pr-manager-helper': { 'type': 'PrManagerHelperProperties',
'if': 'CONFIG_LINUX' },
'qtest': 'QtestProperties',
new file mode 100644
@@ -0,0 +1,123 @@
+/*
+ * QEMU host POSIX shared memory object backend
+ *
+ * Copyright (C) 2024 Red Hat Inc
+ *
+ * Authors:
+ * Stefano Garzarella <sgarzare@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#include "qemu/osdep.h"
+#include "sysemu/hostmem.h"
+#include "qapi/error.h"
+
+#define TYPE_MEMORY_BACKEND_SHM "memory-backend-shm"
+
+OBJECT_DECLARE_SIMPLE_TYPE(HostMemoryBackendShm, MEMORY_BACKEND_SHM)
+
+struct HostMemoryBackendShm {
+ HostMemoryBackend parent_obj;
+};
+
+static bool
+shm_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
+{
+ g_autoptr(GString) shm_name = g_string_new(NULL);
+ g_autofree char *backend_name = NULL;
+ uint32_t ram_flags;
+ int fd, oflag;
+ mode_t mode;
+
+ if (!backend->size) {
+ error_setg(errp, "can't create shm backend with size 0");
+ return false;
+ }
+
+ if (!backend->share) {
+ error_setg(errp, "can't create shm backend with `share=off`");
+ return false;
+ }
+
+ /*
+ * Let's use `mode = 0` because we don't want other processes to open our
+ * memory unless we share the file descriptor with them.
+ */
+ mode = 0;
+ oflag = O_RDWR | O_CREAT | O_EXCL;
+ backend_name = host_memory_backend_get_name(backend);
+
+ /*
+ * Some operating systems allow creating anonymous POSIX shared memory
+ * objects (e.g. FreeBSD provides the SHM_ANON constant), but this is not
+ * defined by POSIX, so let's create a unique name.
+ *
+ * From Linux's shm_open(3) man-page:
+ * For portable use, a shared memory object should be identified
+ * by a name of the form /somename;"
+ */
+ g_string_printf(shm_name, "/qemu-" FMT_pid "-shm-%s", getpid(),
+ backend_name);
+
+ fd = shm_open(shm_name->str, oflag, mode);
+ if (fd < 0) {
+ error_setg_errno(errp, errno,
+ "failed to create POSIX shared memory");
+ return false;
+ }
+
+ /*
+ * We have the file descriptor, so we no longer need to expose the
+ * POSIX shared memory object. However it will remain allocated as long as
+ * there are file descriptors pointing to it.
+ */
+ shm_unlink(shm_name->str);
+
+ if (ftruncate(fd, backend->size) == -1) {
+ error_setg_errno(errp, errno,
+ "failed to resize POSIX shared memory to %" PRIu64,
+ backend->size);
+ close(fd);
+ return false;
+ }
+
+ ram_flags = RAM_SHARED;
+ ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
+
+ return memory_region_init_ram_from_fd(&backend->mr, OBJECT(backend),
+ backend_name, backend->size,
+ ram_flags, fd, 0, errp);
+}
+
+static void
+shm_backend_instance_init(Object *obj)
+{
+ HostMemoryBackendShm *m = MEMORY_BACKEND_SHM(obj);
+
+ MEMORY_BACKEND(m)->share = true;
+}
+
+static void
+shm_backend_class_init(ObjectClass *oc, void *data)
+{
+ HostMemoryBackendClass *bc = MEMORY_BACKEND_CLASS(oc);
+
+ bc->alloc = shm_backend_memory_alloc;
+}
+
+static const TypeInfo shm_backend_info = {
+ .name = TYPE_MEMORY_BACKEND_SHM,
+ .parent = TYPE_MEMORY_BACKEND,
+ .instance_init = shm_backend_instance_init,
+ .class_init = shm_backend_class_init,
+ .instance_size = sizeof(HostMemoryBackendShm),
+};
+
+static void register_types(void)
+{
+ type_register_static(&shm_backend_info);
+}
+
+type_init(register_types);
@@ -13,6 +13,7 @@ system_ss.add([files(
if host_os != 'windows'
system_ss.add(files('rng-random.c'))
system_ss.add(files('hostmem-file.c'))
+ system_ss.add([files('hostmem-shm.c'), rt])
endif
if host_os == 'linux'
system_ss.add(files('hostmem-memfd.c'))
@@ -5238,6 +5238,22 @@ SRST
The ``share`` boolean option is on by default with memfd.
+ ``-object memory-backend-shm,id=id,merge=on|off,dump=on|off,share=on|off,prealloc=on|off,size=size,host-nodes=host-nodes,policy=default|preferred|bind|interleave``
+ Creates a POSIX shared memory backend object, which allows
+ QEMU to share the memory with an external process (e.g. when
+ using vhost-user).
+
+ ``memory-backend-shm`` is a more portable and less featureful version
+ of ``memory-backend-memfd``. It can then be used in any POSIX system,
+ especially when memfd is not supported.
+
+ Please refer to ``memory-backend-file`` for a description of the
+ options.
+
+ The ``share`` boolean option is on by default with shm. Setting it to
+ off will cause a failure during allocation because it is not supported
+ by this backend.
+
``-object iommufd,id=id[,fd=fd]``
Creates an iommufd backend which allows control of DMA mapping
through the ``/dev/iommu`` device.