Re: [Qemu-devel] RFC: Partial workaround for buggy guest virtio-balloon driver

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

On Thu, Nov 08, 2012 at 07:11:13AM -0600, Anthony Liguori wrote:
> David Gibson <david@gibson.dropbear.id.au> writes:
> 
> > Linux kernel commits 1a87228f5f1d316002c7c161316f5524592be766
> > "virtio_balloon: Fix endian bug" and
> > 3ccc9372ed0fab33d20f10be3c1efd5776ff5913 "virtio_balloon: fix handling
> > of PAGE_SIZE != 4k" fixed two serious bugs in their (guest side)
> > handling of the virtio balloon.  In practice, these bugs only affected
> > powerpc guests, which is big-endian and frequently configured for 64k
> > base page size.  Attempting to use the balloon with the buggy guest
> > would usually result in an immediate guest crash.
> 
> You should create a new feature VIRTIO_BALLOON_F_ENDIAN_SAFE,
> advertise it in the host, and add a guest kernel patch to ack it in
> newer kernels.
> 
> Older kernels won't ack this feature which gives you a safe way to to
> disable the driver on a big endian host.

Well, yes, we should have done this at the time we made the bugfixes.
Unfortunately, we didn't, so now we are where we are.

> You won't get support for 3.4 kernels but it's much nicer to handle it
> this way.

Since 3.4 and 3.5 kernels will be around in distros for some time now,
that is not a trivial drawback with adding a feature now.

-- 
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_!
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