backends/hostmem-shm: factor out allocation of "anonymous shared memory with an fd"

Let's factor it out so we can reuse it.

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/1736967650-129648-2-git-send-email-steven.sistare@oracle.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>

backends/hostmem-shm.c

@@ -25,11 +25,9 @@ struct HostMemoryBackendShm {
 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;
+    int fd;
-    mode_t mode;
 
     if (!backend->size) {
         error_setg(errp, "can't create shm backend with size 0");
@@ -41,48 +39,13 @@ shm_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
         return false;
     }
 
-    /*
+    fd = qemu_shm_alloc(backend->size, errp);
-     * 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;
     }
 
+    /* Let's do the same as memory-backend-ram,share=on would do. */
+    backend_name = host_memory_backend_get_name(backend);
     ram_flags = RAM_SHARED;
     ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
 

include/qemu/osdep.h

@@ -509,6 +509,7 @@ int qemu_daemon(int nochdir, int noclose);
 void *qemu_anon_ram_alloc(size_t size, uint64_t *align, bool shared,
                           bool noreserve);
 void qemu_anon_ram_free(void *ptr, size_t size);
+int qemu_shm_alloc(size_t size, Error **errp);
 
 #ifdef _WIN32
 #define HAVE_CHARDEV_SERIAL 1

meson.build

@@ -3696,9 +3696,13 @@ libqemuutil = static_library('qemuutil',
                              build_by_default: false,
                              sources: util_ss.sources() + stub_ss.sources() + genh,
                              dependencies: [util_ss.dependencies(), libm, threads, glib, socket, malloc])
+qemuutil_deps = [event_loop_base]
+if host_os != 'windows'
+  qemuutil_deps += [rt]
+endif
 qemuutil = declare_dependency(link_with: libqemuutil,
                               sources: genh + version_res,
-                              dependencies: [event_loop_base])
+                              dependencies: qemuutil_deps)
 
 if have_system or have_user
   decodetree = generator(find_program('scripts/decodetree.py'),
@@ -4357,7 +4361,7 @@ if have_tools
   subdir('contrib/elf2dmp')
 
   executable('qemu-edid', files('qemu-edid.c', 'hw/display/edid-generate.c'),
-             dependencies: qemuutil,
+             dependencies: [qemuutil, rt],
              install: true)
 
   if have_vhost_user

util/oslib-posix.c

@@ -931,3 +931,55 @@ void qemu_close_all_open_fd(const int *skip, unsigned int nskip)
         qemu_close_all_open_fd_fallback(skip, nskip, open_max);
     }
 }
+
+int qemu_shm_alloc(size_t size, Error **errp)
+{
+    g_autoptr(GString) shm_name = g_string_new(NULL);
+    int fd, oflag, cur_sequence;
+    static int sequence;
+    mode_t mode;
+
+    cur_sequence = qatomic_fetch_inc(&sequence);
+
+    /*
+     * 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;
+
+    /*
+     * 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-%d", getpid(),
+                    cur_sequence);
+
+    fd = shm_open(shm_name->str, oflag, mode);
+    if (fd < 0) {
+        error_setg_errno(errp, errno,
+                         "failed to create POSIX shared memory");
+        return -1;
+    }
+
+    /*
+     * 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, size) == -1) {
+        error_setg_errno(errp, errno,
+                         "failed to resize POSIX shared memory to %zu", size);
+        close(fd);
+        return -1;
+    }
+
+    return fd;
+}

util/oslib-win32.c

@@ -877,3 +877,9 @@ void qemu_win32_map_free(void *ptr, HANDLE h, Error **errp)
     }
     CloseHandle(h);
 }
+
+int qemu_shm_alloc(size_t size, Error **errp)
+{
+    error_setg(errp, "Shared memory is not supported.");
+    return -1;
+}