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

[Amit Shah]

On (Fri) 28 Nov 2014 [11:50:51], David Gibson wrote:
> 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.

Thanks.

                Amit