From cda174ad842c9a61bc275315bf3155139ba19bc0 Mon Sep 17 00:00:00 2001
From: John Snow <jsnow@redhat.com>
Date: Thu, 11 May 2017 20:53:51 +0200
Subject: [PATCH 1/5] blockdev: ignore aio=native for empty drives

RH-Author: John Snow <jsnow@redhat.com>
Message-id: <20170511205351.6337-2-jsnow@redhat.com>
Patchwork-id: 75070
O-Subject: [RHV-7.4 qemu-kvm-rhev PATCH 1/1] blockdev: ignore aio=native for empty drives
Bugzilla: 1402645
RH-Acked-by: Kevin Wolf <kwolf@redhat.com>
RH-Acked-by: Eric Blake <eblake@redhat.com>
RH-Acked-by: Max Reitz <mreitz@redhat.com>

This is a bit of a gross one; Upstream QEMU changed the way it handles
cache options with regards to removable media, associating options more
with the medium instead of the device. As part of that, it became
impossible to specify cache options on empty drives.

In the future, one would use blockdev-add instead to choose cache options
per-medium instead of per-device, but we're not there yet in libvirt so
we added a workaround downstream to simply ignore cache options on empty
CDROMs under the premise of "It actually never worked anyway."

As fallout from this decision, it is now no longer possible to specify
aio=native on empty CDROM devices either, as that requires the use of a
cache option that you can no longer specify. As a bad, gross, disgusting
workaround, simply ignore aio=native on empty drives until such time that
libvirt stops providing such configurations.

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
---
 blockdev.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/blockdev.c b/blockdev.c
index a52e035..033c3fd 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -474,6 +474,21 @@ static bool __redhat_com_has_bs_opts(QDict *bs_opts)
     return s != n;
 }
 
+/**
+ * libvirt expects to be able to pass io driver options (aio=native) for CDROM
+ * drives without inserted media. While this has worked historically, given the
+ * above workaround and lack of a supported alternative in current versions of
+ * libvirt, certain options such as aio=native cannot be supported as it
+ * requires the use of an accompanying cache option, which we also ignore.
+ * Until libvirt learns how to supply cache options to inserted media, ignore
+ * the aio= preference on empty CDROMs with the understanding that un-tuned
+ * performance is preferable to being unable to use the CDROM at all.
+ */
+static int __redhat_com_filter_flags(int flags)
+{
+    return flags & ~BDRV_O_NATIVE_AIO;
+}
+
 /* Takes the ownership of bs_opts */
 static BlockBackend *blockdev_init(const char *file, QDict *bs_opts,
                                    Error **errp)
@@ -586,7 +601,7 @@ static BlockBackend *blockdev_init(const char *file, QDict *bs_opts,
 
         blk = blk_new(0, BLK_PERM_ALL);
         blk_rs = blk_get_root_state(blk);
-        blk_rs->open_flags    = bdrv_flags;
+        blk_rs->open_flags    = __redhat_com_filter_flags(bdrv_flags);
         blk_rs->read_only     = read_only;
         blk_rs->detect_zeroes = detect_zeroes;
 
-- 
1.8.3.1