[PATCH 18/19] arm64: kdump: update a kernel doc

[Ard Biesheuvel]

On 20 January 2016 at 13:36, Mark Rutland <mark.rutland at arm.com> wrote:
> Ard, Ganapatrao, the below is something we need to consider for the
> combination of the NUMA & kexec approaches. It only becomes a problem
> if/when we preserve DT memory nodes in the presence of EFI, though it
> would be nice to not box ourselves into a corner.
>
> On Wed, Jan 20, 2016 at 12:02:58PM +0000, Mark Rutland wrote:
>> On Wed, Jan 20, 2016 at 02:25:07PM +0900, AKASHI Takahiro wrote:
>> > On 01/19/2016 11:01 PM, Mark Rutland wrote:
>> > >For NUMA topology in !ACPI kernels, we might need to also retain and
>> > >parse memory nodes, but only for toplogy information. The kernel would
>> > >still only use memory as described by the EFI memory map.
>> > >
>> > >There's a horrible edge case I've spotted if performing a chain of
>> > >cross-endian kexecs: LE -> BE -> LE, as the BE kernel would have to
>> > >respect the EFI memory map so as to avoid corrupting it for the
>> > >subsequent LE kernel. Other than this I believe everything should just
>> > >work.
>> >
>> > BE kernel doesn't support UEFI yet and cannot access UEFI memmap table. So,
>> > for LE -> BE, we don't use a dtb generated from /sys/firmware/fdt (or /proc/device-tree)
>> > (as in the case of LE -> LE) and require users to provide a dtb file explicitly.
>>
>> As I mentioned above, the problem exists when memory nodes also exist
>> (for describing NUMA topology). In that case the BE kernel would try to
>> use the information from the memory nodes.
>>
>> > For BE -> LE, BE kernel doesn't know wther UEFI memmap table is available or not
>> > and so use the same (explicitly-provided) dtb (as LE -> LE in !UEFI)
>>
>> See above. The problem I imagine is:
>>
>> LE kernel - uses EFI mmap, takes NUMA information from DT memory nodes
>>
>>     v       kexec
>>
>> BE kernel - uses DT memory nodes
>>           - clobbers EFI runtime regions as it sees them as available
>>
>>     v       kexec
>>
>> LE kernel - uses EFI mmap, takes NUMA information from DT memory nodes
>>           - tries to call EFI runtime services, and explodes.
>
> I'm not really sure what the best approach is here, but I thought that
> it would be good to raise awareness of the edge-case.
>

I think we should simply allow the BE kernel to deal with a UEFI
memory map. It only involves a bit of byte swapping (which I already
implemented at some point)

It would require some minor refactoring to make the UEFI init code
separate from all the other bits, but I don't see any major issues
here