Bug 1077302 - Disable host_cdrom detection logic, fall back to plain file: protocol
Summary: Disable host_cdrom detection logic, fall back to plain file: protocol
Alias: None
Product: Red Hat Enterprise Linux 7
Classification: Red Hat
Component: qemu-kvm
Version: 7.0
Hardware: Unspecified
OS: Unspecified
Target Milestone: rc
: ---
Assignee: Markus Armbruster
QA Contact: Virtualization Bugs
Depends On:
TreeView+ depends on / blocked
Reported: 2014-03-17 16:38 UTC by Cole Robinson
Modified: 2015-06-09 16:30 UTC (History)
14 users (show)

Fixed In Version:
Doc Type: Bug Fix
Doc Text:
Clone Of:
Last Closed: 2014-07-16 13:22:13 UTC
Target Upstream Version:

Attachments (Terms of Use)

Description Cole Robinson 2014-03-17 16:38:52 UTC
host_cdrom support is compiled out of qemu, for good reasons, as mentioned here:


Though the implementation means that any attempt to use /dev/sr0 through libvirt will fail. /dev/sr0 is detected as host_cdrom, qemu reports host_cdrom isn't available, and fails.

This breaks some prominent bits of virt-manager and boxes UI that expect to be able to attach a host cdrom device to a guest. There are bugs for that:


However it seems like less work and friendly for our users to just have qemu fall back to treating /dev/sr0 like it would any other block device, and not use the special host_cdrom integration.

This patch works for me, but may not be suitable as is:

diff --git a/block/raw-posix.c b/block/raw-posix.c
index 1688e16..7802724 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -2028,28 +2028,7 @@ static int cdrom_open(BlockDriverState *bs, QDict *option
 static int cdrom_probe_device(const char *filename)
-    int fd, ret;
-    int prio = 0;
-    struct stat st;
-    fd = qemu_open(filename, O_RDONLY | O_NONBLOCK);
-    if (fd < 0) {
-        goto out;
-    }
-    ret = fstat(fd, &st);
-    if (ret == -1 || !S_ISBLK(st.st_mode)) {
-        goto outc;
-    }
-    /* Attempt to detect via a CDROM specific ioctl */
-    ret = ioctl(fd, CDROM_DRIVE_STATUS, CDSL_CURRENT);
-    if (ret >= 0)
-        prio = 100;
-    qemu_close(fd);
-    return prio;
+    return 0;

This may not solve the libvirt + /dev/sr0 + udev + selinux relabelling issues as linked in the host_cdrom bug report, I'm not sure (but they may have been due to a side effect of host_cdrom behavior, so perhaps they will be avoided). There may well be other reasons, but I think something along the above idea should be considered.

Comment 1 Markus Armbruster 2014-06-24 13:31:29 UTC
The patch makes qemu-kvm use backend "host_device" instead of

I have two problems with this idea:

1. It turns "Attempt to use host CD-ROM is rejected out of hand with a
reasonably clear error message" into "Host CD-ROM is accepted, but
doesn't fully work".  In particular, guest commands to eject and load
media succeed without doing anything.  I doubt that's an improvement.

2. The boat has sailed: we shipped 7.0.  I'm rather reluctant to
change how we treat host CD-ROM now.  Feedback from customers could
overcome my reluctance.

Any objections to WONTFIX?

Comment 2 Markus Armbruster 2014-07-16 13:22:13 UTC
Closing WONTFIX.  If you disagree, please reopen stating your arguments.

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