Re: [PATCH] x86/hvm/dom0: fix PVH initrd and metadata placement

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.10.2023 12:31, Andrew Cooper wrote:
> On 26/10/2023 9:34 am, Xenia Ragiadakou wrote:
>> On 26/10/23 10:35, Jan Beulich wrote:
>>> On 26.10.2023 08:45, Xenia Ragiadakou wrote:
>>>> Given that start < kernel_end and end > kernel_start, the logic that
>>>> determines the best placement for dom0 initrd and metadata, does not
>>>> take into account the two cases below:
>>>> (1) start > kernel_start && end > kernel_end
>>>> (2) start < kernel_start && end < kernel_end
>>>>
>>>> In case (1), the evaluation will result in end = kernel_start
>>>> i.e. end < start, and will load initrd in the middle of the kernel.
>>>> In case (2), the evaluation will result in start = kernel_end
>>>> i.e. end < start, and will load initrd at kernel_end, that is out
>>>> of the memory region under evaluation.
>>> I agree there is a problem if the kernel range overlaps but is not fully
>>> contained in the E820 range under inspection. I'd like to ask though
>>> under what conditions that can happen, as it seems suspicious for the
>>> kernel range to span multiple E820 ranges.
>>
>> We tried to boot Zephyr as pvh dom0 and its load address was under 1MB.
>>
>> I know ... that maybe shouldn't have been permitted at all, but
>> nevertheless we hit this issue.
> 
> 
> Zephyr is linked to run at 4k.  That's what the ELF Headers say, and the
> entrypoint is not position-independent.

Very interesting. What size is their kernel? And, Xenia, can you provide
the E820 map that you were finding the collision with?

Jan

> So trying to put the binary anywhere else is going to work about as well
> having the notes section misalign the pagetables by 0x20 bytes[1].
> 
> ~Andrew
> 
> [1] Guess what was causing the "Zephyr doesn't boot PVH" issues.  Which
> is doubly irritating because about 6h of debugging prior, I'd pointed
> out that the linker was complaining about an orphaned section and that
> that ought to be fixed before trying to debug further...