3 files changed, 71 insertions, 3 deletions

diff --git a/util/memfd.c b/util/memfd.c
index 8a2e906962..07beab174d 100644
--- a/util/memfd.c
+++ b/util/memfd.c
@@ -194,17 +194,27 @@ bool qemu_memfd_alloc_check(void)
 /**
  * qemu_memfd_check():
  *
- * Check if host supports memfd.
+ * Check if host supports memfd.  Cache the answer for the common case flags=0.
  */
 bool qemu_memfd_check(unsigned int flags)
 {
 #ifdef CONFIG_LINUX
-    int mfd = memfd_create("test", flags | MFD_CLOEXEC);
+    int mfd;
+    static int memfd_check = MEMFD_TODO;
+
+    if (!flags && memfd_check != MEMFD_TODO) {
+        return memfd_check;
+    }
 
+    mfd = memfd_create("test", flags | MFD_CLOEXEC);
     if (mfd >= 0) {
         close(mfd);
-        return true;
     }
+    if (!flags) {
+        memfd_check = (mfd >= 0) ? MEMFD_OK : MEMFD_KO;
+    }
+    return (mfd >= 0);
+
 #endif
 
     return false;
diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index 7a542cb50b..2bb34dade3 100644
--- a/util/oslib-posix.c
+++ b/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;
+}
diff --git a/util/oslib-win32.c b/util/oslib-win32.c
index b623830d62..b7351634ec 100644
--- a/util/oslib-win32.c
+++ b/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;
+}