Re: [Xen-devel] Xen 4.1 + Linux compiled with PVH == BOOM

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

If once the kernel starts running, it (for some reason) is unable to
boot in PV mode when it has PVH also compiled in, then this is certainly
a kernel bug.

~Andrew

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