backends/hostmem-shm.c

   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
  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
  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
 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
 118 static void register_types(void)
 119 {
 120     type_register_static(&shm_backend_info);
 121 }
 122
 123 type_init(register_types);