Why has CD/DVD host passthrough been disabled in CentOS 7 \ RHEL 7?

Question

I'm learning to use the KVM on CentOS 7 right now. For some reason Red Hat decided to disable the possibility of installing an OS on a virtual machine from a CD\DVD-Rom in RHEL 7. When creating a new VM through the VM Manager the "Use CDROM or DVD" option is greyed out and unavailable and the hint says "Physical CDROM passthrough not supported with this hypervisor". I found a couple of bug descriptions related to the issue, but all of them have to do with ramifications of CDROM passthrough disabling and neither describe the reason of why it was disabled in the first place. The bugs are from Red Hat's bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1072294 https://bugzilla.redhat.com/show_bug.cgi?id=1072610

Both of them refer to 760885 bug that apparently describes the reason. Unfortunately it isn't made public and access is denied even after registration.

Can anyone shed a light on why they have disabled such a seemingly useful feature? Is it a security or functionality issue?

Thank you, golem

Solution

Disclaimer: I work for Red Hat but have no idea about the decisions related to this particular bug. I made a request, though, and the bug is now public.

The short version is: passthrough seems to have a lot of bugs in different situations, none of them simple fixes, and it (again, really not speaking officially here) seems like it was judged to not really be good to expose to users. (For example, https://bugzilla.redhat.com/show_bug.cgi?id=729219)

However, there is work on fixing many of those, and it may be re-enabled at some nebulous point in the future. (But I do suggest using the suggestion above to make an file image from your CDROM and using that.)

Comments

[Michael Hampton]

I don't know why this was done, but I can say I haven't actually needed to do it in many years. Using images is much more convenient.

[David Corsalini]

Turn the cdrom into an image using dd if=/dev/cdrom of=/path/to/image.iso and attach the ISO.

[Michael Hampton]

760885 is readable now, but it probably doesn't help much.