Re: [Qemu-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes.

[Alex Bligh]

--On 18 March 2013 14:32:23 +0100 Paolo Bonzini <address@hidden> wrote:

> How does migration work with this change?  The target may read stale
> data from the page cache.  Fix the kernel bug please.

Presumably the same way as if writeback caching is selected. I presume
that must fsync() / fdatasync() all the data to disk, and a barrier will
produce one of those.

It would be great to fix the kernel bug (and I have submitted code), but
the fix is pretty intrusive (see the link I posted) and there appears
to be little interest in taking it forward. Certainly my kernel hacking
skills are not adequate to the task.

The current position is that booting a Xen domU which does disk I/O
(Ubuntu cloud image used as the test case) with an NFS root crashes dom0
absolutely repeatably, and kills all other guests. Unless and until
there is a kernel fix for that, Xen is in essence unusable with HVM
and network based disk backend. So we need a workaround in the meantime
which doesn't require a kernel fix.

--
Alex Bligh