[PATCH] xen-block: fix reporting of discard feature

[roger.pau--- via <qemu-devel@nongnu.org>]

Linux blkfront expects both "discard-granularity" and
"discard-alignment" present on xenbus in order to properly enable the
feature, not exposing "discard-alignment" left some Linux blkfront
versions with a broken discard setup. This has also been addressed in
Linux with:

https://lore.kernel.org/lkml/20210118151528.81668-1-roger.pau@citrix.com/T/#u

Fix QEMU to report a "discard-alignment" of 0, in order for it to work
with older Linux frontends.

Reported-by: Arthur Borsboom <arthurborsboom@gmail.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Paul Durrant <paul@xen.org>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Cc: xen-devel@lists.xenproject.org
Cc: qemu-block@nongnu.org
---
 hw/block/xen-block.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/hw/block/xen-block.c b/hw/block/xen-block.c
index 718d886e5c..246d9c23a2 100644
--- a/hw/block/xen-block.c
+++ b/hw/block/xen-block.c
@@ -253,6 +253,7 @@ static void xen_block_realize(XenDevice *xendev, Error **errp)
         xen_device_backend_printf(xendev, "feature-discard", "%u", 1);
         xen_device_backend_printf(xendev, "discard-granularity", "%u",
                                   conf->discard_granularity);
+        xen_device_backend_printf(xendev, "discard-alignment", "%u", 0);
     }
 
     xen_device_backend_printf(xendev, "feature-flush-cache", "%u", 1);
-- 
2.29.2