Re: [Xen-devel] [PATCH v2] introduce a cache options for PV disks

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Here's the result.  I took Anthony Ligouri's ack off it.

Ian.

commit 21538601f7d4f2de4d518b92ec9212c99be829e8
Author: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Date:   Thu Jun 27 14:20:51 2013 +0000

    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.
    
    Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>

diff --git a/hw/block/xen_disk.c b/hw/block/xen_disk.c
index 247f32f..091284b 100644
--- a/hw/block/xen_disk.c
+++ b/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(directiofsafe));
 
     /* 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;

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel