Re: Xen 4.1 + Linux compiled with PVH == BOOM

[David Vrabel <dvrabel@cantab.net>]

On 24/12/2013 12:55, Andrew Cooper wrote:
> On 24/12/2013 12:31, David Vrabel wrote:
>> On 20/12/2013 17:57, Konrad Rzeszutek Wilk wrote:
>>> Hey,
>>>
>>> This is with Linux and
>>>
>>> git://git.kernel.org/pub/scm/linux/kernel/git/konrad/xen.git stable/pvh.v11
>>>
>>> I get Xen 4.1 (only) hypervisor to blow up with a Linux kernel that has been
>>> compiled with PVH.
>>>
>>> I think the same problem would show up if I tried to launch a PV guest 
>>> compiled as PVH under Xen 4.1 as well - as the ELF parsing code is shared
>>> with the toolstack.
>> If a kernel with both PVH and PV support enabled cannot boot in PV mode
>> with a non-PVH aware hypervisor/toolstack then the kernel is broken.
>>
>> Hypervisor/tool-side fixes aren't the correct fix here.  Xen 4.1 and
>> even older are still widely deployed.
>>
>> David
> 
> I believe that the problem is because the elf parsing code is not
> sufficiently forward-compatible aware, and rejects the PVH kernel
> because it has an unrecognised Xen elf note field.  This is not a kernel
> bug.
> 
> The elf parsing should accept unrecognised fields for forward
> compatibility, which would then allow a PV & PVH compiled kernel to run
> in PV mode.

It should but it doesn't, so a different way needs to be found for the
kernel to report (optional) PVH support.  A method that is compatible
with older toolstacks.

David