[libvirt PATCH] libxl: Fix build with recent Xen that introduces new disk backend type

[Oleksandr Tyshchenko]

From: Oleksandr Tyshchenko <oleksandr_tyshchenko at epam.com>

Xen toolstack has gained basic Virtio support recently which becides
adding various virtio related stuff introduces new disk backend type
LIBXL_DISK_BACKEND_STANDALONE [1].

Unfortunately, this caused a regression in libvirt build with Xen support
enabled, reported by the osstest today [2]:

CC       libxl/libvirt_driver_libxl_impl_la-xen_xl.lo
../../src/libxl/xen_xl.c: In function 'xenParseXLDisk':
../../src/libxl/xen_xl.c:779:17: error: enumeration value 'LIBXL_DISK_BACKEND_STANDALONE'
   not handled in switch [-Werror=switch-enum]
                 switch (libxldisk->backend) {
                 ^~~~~~
cc1: all warnings being treated as errors

The interesting fact is that switch already has a default branch (which ought
to cover such new addition), but the error is triggered as -Wswitch-enum
gives a warning about an omitted enumeration code even if there is a default
label.

Also there is a similar issue in libxlUpdateDiskDef() which I have reproduced
after fixing the first one, but it that case the corresponding switch doesn't
have a default branch.

Fix both issues by inserting required enumeration item to make the compiler
happy and adding ifdef guard to be able to build against old Xen libraries
as well (without LIBXL_HAVE_DEVICE_DISK_SPECIFICATION). Also add a default
branch to switch in libxlUpdateDiskDef().

Please note, that current patch doesn't implement the proper handling of
LIBXL_DISK_BACKEND_STANDALONE and friends, it is just intended to fix
the regression immediately to unblock the osstest.  Also it worth mentioning
that current patch won't solve the possible additions in the future.

[1] https://lore.kernel.org/xen-devel/20220716163745.28712-1-olekstysh@gmail.com/
[2] https://lore.kernel.org/xen-devel/E1oHEQO-0008GA-Uo@osstest.test-lab.xenproject.org/

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko at epam.com>
---
Cc: Julien Grall <julien at xen.org>
Cc: Anthony PERARD <anthony.perard at citrix.com>
Cc: Michal Privoznik <mprivozn at redhat.com>

Please note, the patch is tested on:
https://xenbits.xen.org/gitweb/?p=libvirt.git;a=shortlog;h=refs/heads/xen-tested-master
but should work on the master as well (as the same code is present here).
---
 src/libxl/libxl_conf.c | 4 ++++
 src/libxl/xen_xl.c     | 3 +++
 2 files changed, 7 insertions(+)

diff --git a/src/libxl/libxl_conf.c b/src/libxl/libxl_conf.c
index aa3d7925ec..526f0b2b08 100644
--- a/src/libxl/libxl_conf.c
+++ b/src/libxl/libxl_conf.c
@@ -1240,6 +1240,10 @@ libxlUpdateDiskDef(virDomainDiskDef *l_disk, libxl_device_disk *x_disk)
         driver = "phy";
         break;
     case LIBXL_DISK_BACKEND_UNKNOWN:
+#ifdef LIBXL_HAVE_DEVICE_DISK_SPECIFICATION
+    case LIBXL_DISK_BACKEND_STANDALONE:
+#endif
+    default:
         break;
     }
     if (driver)
diff --git a/src/libxl/xen_xl.c b/src/libxl/xen_xl.c
index 4de4e3140f..6919325623 100644
--- a/src/libxl/xen_xl.c
+++ b/src/libxl/xen_xl.c
@@ -715,6 +715,9 @@ xenParseXLDisk(virConf *conf, virDomainDef *def)
                     virDomainDiskSetDriver(disk, "phy");
                     virDomainDiskSetType(disk, VIR_STORAGE_TYPE_BLOCK);
                     break;
+#ifdef LIBXL_HAVE_DEVICE_DISK_SPECIFICATION
+                case LIBXL_DISK_BACKEND_STANDALONE:
+#endif
                 default:
                     virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
                                    _("disk backend not supported: %s"),
-- 
2.25.1