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[Xen-devel] [PATCH]: Make Xen 3.1 IDE flush on O_DIRECT with drive caching off

  • To: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: Chris Lalancette <clalance@xxxxxxxxxx>
  • Date: Tue, 26 Feb 2008 09:41:30 -0500
  • Delivery-date: Tue, 26 Feb 2008 06:41:56 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>

     Long ago Xen added code to the device model to basically do an fsync()
after every data write if the user in the guest specified that IDE write caching
should be disabled.  This works fine, except in the case where you are doing
O_DIRECT writes inside the guest (ala dd if=/dev/zero of=/dev/hdb oflag=direct).
 This is because you can get out of ide_write_dma_cb() in the middle of the loop
without going through the logic to sync.  This simple patch makes sure that you
always check (and sync) inside the write callback.

This patch applies to xen-3.1-testing.hg; I still have to test 3.2 (and the new
AIO code) to see if it is affected by this bug.

Signed-off-by: Chris Lalancette <clalance@xxxxxxxxxx>
--- xen-3.1.0-src/tools/ioemu/hw/ide.c.orig     2008-02-25 06:46:26.000000000 
+++ xen-3.1.0-src/tools/ioemu/hw/ide.c  2008-02-25 06:46:30.000000000 -0500
@@ -1024,7 +1024,9 @@ static int ide_write_dma_cb(IDEState *s,
 #else  /* !DMA_MULTI_THREAD */
 #endif /* DMA_MULTI_THREAD */
-                return 0;
+               /* make sure that we (possibly) flush before leaving */
+               break;
             if (n > MAX_MULT_SECTORS)
                 n = MAX_MULT_SECTORS;
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