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[Xen-devel] [PATCH for 4.5] libxl: Tell qemu to use raw format when using a tapdisk

At the moment libxl unconditinally passes the underlying file format
to qemu in the device string.  However, when tapdisk is in use,
tapdisk handles the underlying format and presents qemu with
effectively a raw disk.  When qemu looks at the tapdisk block device
and doesn't find the image format it was looking for, it will fail.

This effectively means that tapdisk cannot be used with HVM domains at
the moment except for raw files.

Instead, if we're using a tapdisk backend, tell qemu to use a raw file

Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
CC: Ian Campbell <ian.campbell@xxxxxxxxxx>
CC: Ian Jackson <ian.jackson@xxxxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Konrad Wilk <konrad.wilk@xxxxxxxxxx>

Release exception justification: This fixes a bug in functionality, in
that at the moment HVM guests cannot boot with tapdisk and vhd format.

This is not a regression in xl functionality per se, since (AFAICT)
this has never worked.  However, given that 4.5 is the first release
without xend, this *does* represent a regression in functionality for
Xen as a whole (since before people using hvm guest with vhd on blktap
could use xend).

The fix is very simple and should only affect codepaths that already
don't work, so the risk of regressions should be very low.

While preparing this patch, I also noticed that cdroms will ignore the
backend parameter and treat everything as a file.  This is a bug but I
think it's a much less important one to address this late in the
release cycle.
 tools/libxl/libxl_dm.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
index b25b574..10f3090 100644
--- a/tools/libxl/libxl_dm.c
+++ b/tools/libxl/libxl_dm.c
@@ -797,11 +797,14 @@ static char ** 
libxl__build_device_model_args_new(libxl__gc *gc,
-                if (disks[i].backend == LIBXL_DISK_BACKEND_TAP)
+                if (disks[i].backend == LIBXL_DISK_BACKEND_TAP) {
+                    format = qemu_disk_format_string(LIBXL_DISK_FORMAT_RAW);
                     pdev_path = libxl__blktap_devpath(gc, disks[i].pdev_path,
-                else
+                } else {
                     pdev_path = disks[i].pdev_path;
+                }
                  * Explicit sd disks are passed through as is.

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