[Xen-users] xl v4.6 and qemu 2.6: Failure to create HVM domains with empty cdrom drives: "Must specify either driver or file"

[Stephen Oberholtzer <stevie@xxxxxxxxx>]

(creating new thread at request of George Dunlap)

This past weekend I did a massive upgrade of my system: kernel (3.16 -> 4.6), Xen (4.5 -> 4.6), and qemu (2.1 -> 2.6).  After this, my HVM Linux domain booted, but my HVM Windows machine did not.

After some investigation, I discovered in /var/log/xen/qemu-dm-winhvm.log that qemu was choking on the cdrom drive definition.  This quickly revealed that some non-backwards-compatible changes were made to qemu:

https://bugzilla.redhat.com/show_bug.cgi?id=1342999

From reading that bug, the fault appears to lie with qemu:

As best as I can tell, the issue is this: in qemu, the 'cache' option has been switched from being associated to the drive, to being associated to the backing file/device.

When xl passes 'cache=writeback' to qemu, this causes qemu to activate the code path that tries to open a file.  Unfortunately, because there's no file/device, this causes an error.  And, from my reading of the various documentation files, there doesn't seem to be a way to tell xl to omit the 'cache=writeback' parameter.

This does raise an interesting point, though; qemu's documentation states that cache=writeback uses the host's page cache.  Wouldn't that cause double-caching, where data is cached in both the Dom0 cache and in the DomU OS's cache? Wouldn't we want cache=none for just about *any* dedicated DomU storage (unless you were running some crazy non-caching OS.)

--

-- Stevie-O
Real programmers use COPY CON PROGRAM.EXE

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