backends/hostmem-file: Allow to specify full pathname for backing file

This allows to explicitly specify file name to use with the backend. This is important when using it together with ivshmem in order to make it backed by hugetlbfs. By default filename is autogenerated using mkstemp(), and the file is unlink()ed after creation, effectively making it anonymous. This is not very useful with ivshmem because it ends up in a memory which cannot be accessed by something else.

Distinction between directory and file name is done by stat() check. If an existing directory is given, the code keeps old behavior. Otherwise it creates or opens a file with the given pathname.

Signed-off-by Pavel Fedin <[email protected]>

Tested-by Igor Skalkin <[email protected]>

---
v1 => v2:
- Changed title to more generic one
- Do not introduce new property, check whether the given path is a directory instead

---
 exec.c        | 34 +++++++++++++++++++++-------------
 qemu-doc.texi |  2 +-
 2 files changed, 22 insertions(+), 14 deletions(-)

exec.c +21 -13

index 8af2570..3238c9a 100644
@@ -1205,6 +1205,7 @@ static void *file_ram_alloc(RAMBlock *block,
                             const char *path,
                             Error **errp)
 {
+    struct stat st;
     char *filename;
     char *sanitized_name;
     char *c;

@@ -1233,26 +1234,33 @@ static void *file_ram_alloc(RAMBlock *block,
         goto error;
     }
 
-    /* Make name safe to use with mkstemp by replacing '/' with '_'. */
-    sanitized_name = g_strdup(memory_region_name(block->mr));
-    for (c = sanitized_name; *c != '\0'; c++) {
-        if (*c == '/')
-            *c = '_';
-    }
+    if (!stat(path, &st) && S_ISDIR(st.st_mode)) {
+        /* Make name safe to use with mkstemp by replacing '/' with '_'. */
+        sanitized_name = g_strdup(memory_region_name(block->mr));
+        for (c = sanitized_name; *c != '\0'; c++) {
+            if (*c == '/') {
+                *c = '_';
+            }
+        }
 
-    filename = g_strdup_printf("%s/qemu_back_mem.%s.XXXXXX", path,
-                               sanitized_name);
-    g_free(sanitized_name);
+        filename = g_strdup_printf("%s/qemu_back_mem.%s.XXXXXX", path,
+                                   sanitized_name);
+        g_free(sanitized_name);
+
+        fd = mkstemp(filename);
+        if (fd >= 0) {
+            unlink(filename);
+        }
+        g_free(filename);
+    } else {
+        fd = open(path, O_RDWR | O_CREAT, 0644);
+    }
 
-    fd = mkstemp(filename);
     if (fd < 0) {
         error_setg_errno(errp, errno,
                          "unable to create backing store for hugepages");
-        g_free(filename);
         goto error;
     }
-    unlink(filename);
-    g_free(filename);
 
     memory = ROUND_UP(memory, hpagesize);
 

qemu-doc.texi +0 -0

index 3126abd..460ab71 100644