From 89b162019bfd202bbbd00563d03a030c2f7c1395 Mon Sep 17 00:00:00 2001
From: John Snow <jsnow@redhat.com>
Date: Fri, 16 Sep 2016 22:06:28 +0200
Subject: blockdev: ignore cache options for empty CDROM drives

RH-Author: John Snow <jsnow@redhat.com>
Message-id: <1474063588-6370-2-git-send-email-jsnow@redhat.com>
Patchwork-id: 72377
O-Subject: [RHEV-7.3 qemu-kvm-rhev PATCH 1/1] blockdev: ignore cache options for empty CDROM drives
Bugzilla: 1342999
RH-Acked-by: Max Reitz <mreitz@redhat.com>
RH-Acked-by: Kevin Wolf <kwolf@redhat.com>
RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>

In qemu-kvm-rhev-2.3.0, QEMU will accept cache options for empty CDROM
devices, but silently ignore them as they will be overwritten when the
next CDROM is inserted.

Libvirt and VMM are capable of generating XML configurations which
attempt to specify these cache options to QEMU, though they don't have
any effect.

Upstream, a refactoring of cache option mechanisms means that we have
started rejecting invalid configurations where cache options are supplied
without any target to actually apply them to.

This means that there are combinations of QEMU and libvirt that will fail
to start a VM if a user selects a cache option.

This patch is a downstream-only workaround until libvirt can stop
supplying cache settings for empty CDROMs and/or until libvirt can take
advantage of the new QMP tray/medium manipulation mechanisms that will
allow proper cache specification for removable media.

Signed-off-by: John Snow <jsnow@redhat.com>
Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
(cherry picked from commit f9bc62a9e433fb67845477fd76e0c2b8d6685b93)
(cherry picked from commit 018339d32a9393371c1a49ff52e83de7d2ec4910)
---
 blockdev.c | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/blockdev.c b/blockdev.c
index dddf2e1..a52e035 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -448,6 +448,32 @@ static void extract_common_blockdev_options(QemuOpts *opts, int *bdrv_flags,
     }
 }
 
+/**
+ * libvirt expects to be able to pass cache options for CDROM drives without
+ * inserted media. Historically, QEMU eventually ignores these cache options as
+ * they are lost when media is inserted. Recently, QEMU started rejecting these
+ * configurations. Libvirt however still generates such configurations.
+ *
+ * To prevent QEMU from being unable to start, pretend there are no options
+ * present if the only options present are cache options for the BDS.
+ */
+static bool __redhat_com_has_bs_opts(QDict *bs_opts)
+{
+    size_t n, s;
+    s = qdict_size(bs_opts);
+
+    if (s == 0) {
+        return false;
+    } else if (s > 2) {
+        return true;
+    }
+
+    n = qdict_haskey(bs_opts, BDRV_OPT_CACHE_DIRECT);
+    n += qdict_haskey(bs_opts, BDRV_OPT_CACHE_NO_FLUSH);
+
+    return s != n;
+}
+
 /* Takes the ownership of bs_opts */
 static BlockBackend *blockdev_init(const char *file, QDict *bs_opts,
                                    Error **errp)
@@ -555,7 +581,7 @@ static BlockBackend *blockdev_init(const char *file, QDict *bs_opts,
     read_only = qemu_opt_get_bool(opts, BDRV_OPT_READ_ONLY, false);
 
     /* init */
-    if ((!file || !*file) && !qdict_size(bs_opts)) {
+    if ((!file || !*file) && !__redhat_com_has_bs_opts(bs_opts)) {
         BlockBackendRootState *blk_rs;
 
         blk = blk_new(0, BLK_PERM_ALL);
-- 
1.8.3.1