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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: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
This commit is contained in:
Stefano Stabellini 2013-07-29 10:56:38 +00:00
parent 461bdb3414
commit 454ae734f1
1 changed files with 13 additions and 1 deletions
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14
hw/block/xen_disk.c
View File

@ -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;