1 /*
2 * QEMU host POSIX shared memory object backend
3 *
4 * Copyright (C) 2024 Red Hat Inc
5 *
6 * Authors:
7 * Stefano Garzarella <sgarzare@redhat.com>
8 *
9 * This work is licensed under the terms of the GNU GPL, version 2 or later.
10 * See the COPYING file in the top-level directory.
11 */
12
13 #include "qemu/osdep.h"
14 #include "sysemu/hostmem.h"
15 #include "qapi/error.h"
16
17 #define TYPE_MEMORY_BACKEND_SHM "memory-backend-shm"
18
19 OBJECT_DECLARE_SIMPLE_TYPE(HostMemoryBackendShm, MEMORY_BACKEND_SHM)
20
21 struct HostMemoryBackendShm {
22 HostMemoryBackend parent_obj;
23 };
24
25 static bool
shm_backend_memory_alloc(HostMemoryBackend * backend,Error ** errp)26 shm_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
27 {
28 g_autoptr(GString) shm_name = g_string_new(NULL);
29 g_autofree char *backend_name = NULL;
30 uint32_t ram_flags;
31 int fd, oflag;
32 mode_t mode;
33
34 if (!backend->size) {
35 error_setg(errp, "can't create shm backend with size 0");
36 return false;
37 }
38
39 if (!backend->share) {
40 error_setg(errp, "can't create shm backend with `share=off`");
41 return false;
42 }
43
44 /*
45 * Let's use `mode = 0` because we don't want other processes to open our
46 * memory unless we share the file descriptor with them.
47 */
48 mode = 0;
49 oflag = O_RDWR | O_CREAT | O_EXCL;
50 backend_name = host_memory_backend_get_name(backend);
51
52 /*
53 * Some operating systems allow creating anonymous POSIX shared memory
54 * objects (e.g. FreeBSD provides the SHM_ANON constant), but this is not
55 * defined by POSIX, so let's create a unique name.
56 *
57 * From Linux's shm_open(3) man-page:
58 * For portable use, a shared memory object should be identified
59 * by a name of the form /somename;"
60 */
61 g_string_printf(shm_name, "/qemu-" FMT_pid "-shm-%s", getpid(),
62 backend_name);
63
64 fd = shm_open(shm_name->str, oflag, mode);
65 if (fd < 0) {
66 error_setg_errno(errp, errno,
67 "failed to create POSIX shared memory");
68 return false;
69 }
70
71 /*
72 * We have the file descriptor, so we no longer need to expose the
73 * POSIX shared memory object. However it will remain allocated as long as
74 * there are file descriptors pointing to it.
75 */
76 shm_unlink(shm_name->str);
77
78 if (ftruncate(fd, backend->size) == -1) {
79 error_setg_errno(errp, errno,
80 "failed to resize POSIX shared memory to %" PRIu64,
81 backend->size);
82 close(fd);
83 return false;
84 }
85
86 ram_flags = RAM_SHARED;
87 ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
88
89 return memory_region_init_ram_from_fd(&backend->mr, OBJECT(backend),
90 backend_name, backend->size,
91 ram_flags, fd, 0, errp);
92 }
93
94 static void
shm_backend_instance_init(Object * obj)95 shm_backend_instance_init(Object *obj)
96 {
97 HostMemoryBackendShm *m = MEMORY_BACKEND_SHM(obj);
98
99 MEMORY_BACKEND(m)->share = true;
100 }
101
102 static void
shm_backend_class_init(ObjectClass * oc,void * data)103 shm_backend_class_init(ObjectClass *oc, void *data)
104 {
105 HostMemoryBackendClass *bc = MEMORY_BACKEND_CLASS(oc);
106
107 bc->alloc = shm_backend_memory_alloc;
108 }
109
110 static const TypeInfo shm_backend_info = {
111 .name = TYPE_MEMORY_BACKEND_SHM,
112 .parent = TYPE_MEMORY_BACKEND,
113 .instance_init = shm_backend_instance_init,
114 .class_init = shm_backend_class_init,
115 .instance_size = sizeof(HostMemoryBackendShm),
116 };
117
register_types(void)118 static void register_types(void)
119 {
120 type_register_static(&shm_backend_info);
121 }
122
123 type_init(register_types);
124