Re: [Qemu-devel] [PATCH] Fix for crash after migration in virtio-rng on bi-endian targets

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Nov 27, 2014 at 02:38:42PM +0530, Amit Shah wrote:
> On (Thu) 27 Nov 2014 [16:48:10], David Gibson wrote:
> > VirtIO devices now remember which endianness they're operating in in order
> > to support targets which may have guests of either endianness, such as
> > powerpc.  This endianness state is transferred in a subsection of the
> > virtio device's information.
> > 
> > With virtio-rng this can lead to an abort after a loadvm hitting the
> > assert() in virtio_is_big_endian().  This can be reproduced by doing a
> > migrate and load from file on a bi-endian target with a virtio-rng device.
> > The actual guest state isn't particularly important to triggering this.
> > 
> > The cause is that virtio_rng_load_device() calls virtio_rng_process() which
> > accesses the ring and thus needs the endianness.  However,
> > virtio_rng_process() is called via virtio_load() before it loads the
> > subsections.  Essentially the ->load callback in VirtioDeviceClass should
> > only be used for actually reading the device state from the stream, not for
> > post-load re-initialization.
> 
> Agreed.
> 
> > This patch fixes the bug by moving the virtio_rng_process() after the call
> > to virtio_load().  Better yet would be to convert virtio to use vmsd and
> > have the virtio_rng_process() as a post_load callback, but that's a bigger
> > project for another day.
> > 
> > This is bugfix, and should be considered for the 2.2 branch.
> 
> This is undoing most of 3902d49e13c2428bd6381cfdf183103ca4477c1f ,
> added Greg to CC list.
> 
> Did you try this on x86 guests, or with multiple rng devices?

Not so far, I'll see what I can do.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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