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

--On 18 March 2013 14:32:23 +0100 Paolo Bonzini <pbonzini@xxxxxxxxxx> 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

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