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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Aug 02, 2016 at 10:40:05AM -0400, Stephen Oberholtzer wrote:
> (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.)
> 

We removed cache=writeback in Xen 4.7 release.

You can cherry-pick d532f45d94c412d6ee0491cd0b44946373ff2268 in xen.git
to your own tree or request a backport on xen-devel.

Wei.

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

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