xen_disk: support "direct-io-safe" backend option

Support backend option "direct-io-safe".  This is documented as
follows in the Xen backend specification:

 * direct-io-safe
 *      Values:         0/1 (boolean)
 *      Default Value:  0
 *
 *      The underlying storage is not affected by the direct IO memory
 *      lifetime bug.  See:
 *        http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html
 *
 *      Therefore this option gives the backend permission to use
 *      O_DIRECT, notwithstanding that bug.
 *
 *      That is, if this option is enabled, use of O_DIRECT is safe,
 *      in circumstances where we would normally have avoided it as a
 *      workaround for that bug.  This option is not relevant for all
 *      backends, and even not necessarily supported for those for
 *      which it is relevant.  A backend which knows that it is not
 *      affected by the bug can ignore this option.
 *
 *      This option doesn't require a backend to use O_DIRECT, so it
 *      should not be used to try to control the caching behaviour.

Also, BDRV_O_NATIVE_AIO is ignored if BDRV_O_NOCACHE, so clarify the
default flags passed to the qemu block layer.

The original proposal for a "cache" backend option has been dropped
because it was believed too wide, especially considering that at the
moment the backend doesn't have a way to tell the toolstack that it is
capable of supporting it.
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: NIan Jackson <Ian.Jackson@eu.citrix.com>

hw/block/xen_disk.c

@@ -93,6 +93,7 @@ struct XenBlkDev {
     char                *type;
     char                *dev;
     char                *devtype;
+    bool                directiosafe;
     const char          *fileproto;
     const char          *filename;
     int                 ring_ref;
@@ -701,6 +702,7 @@ static int blk_init(struct XenDevice *xendev)
 {
     struct XenBlkDev *blkdev = container_of(xendev, struct XenBlkDev, xendev);
     int info = 0;
+    char *directiosafe = NULL;
 
     /* read xenstore entries */
     if (blkdev->params == NULL) {
@@ -733,6 +735,8 @@ static int blk_init(struct XenDevice *xendev)
     if (blkdev->devtype == NULL) {
         blkdev->devtype = xenstore_read_be_str(&blkdev->xendev, "device-type");
     }
+    directiosafe = xenstore_read_be_str(&blkdev->xendev, "direct-io-safe");
+    blkdev->directiosafe = (directiosafe && atoi(directiosafe));
 
     /* do we have all we need? */
     if (blkdev->params == NULL ||
@@ -760,6 +764,8 @@ static int blk_init(struct XenDevice *xendev)
     xenstore_write_be_int(&blkdev->xendev, "feature-flush-cache", 1);
     xenstore_write_be_int(&blkdev->xendev, "feature-persistent", 1);
     xenstore_write_be_int(&blkdev->xendev, "info", info);
+
+    g_free(directiosafe);
     return 0;
 
 out_error:
@@ -773,6 +779,8 @@ out_error:
     blkdev->dev = NULL;
     g_free(blkdev->devtype);
     blkdev->devtype = NULL;
+    g_free(directiosafe);
+    blkdev->directiosafe = false;
     return -1;
 }
 
@@ -783,7 +791,11 @@ static int blk_connect(struct XenDevice *xendev)
     bool readonly = true;
 
     /* read-only ? */
-    qflags = BDRV_O_CACHE_WB | BDRV_O_NATIVE_AIO;
+    if (blkdev->directiosafe) {
+        qflags = BDRV_O_NOCACHE | BDRV_O_NATIVE_AIO;
+    } else {
+        qflags = BDRV_O_CACHE_WB;
+    }
     if (strcmp(blkdev->mode, "w") == 0) {
         qflags |= BDRV_O_RDWR;
         readonly = false;