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

[takahiro.akashi@linaro.org (AKASHI Takahiro)]

On 01/20/2016 11:59 PM, Ard Biesheuvel wrote:
> On 20 January 2016 at 13:36, Mark Rutland <mark.rutland@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)

Just from my curiosity,
will runtime services be also available on BE kernel with LE uefi?

-Takahiro AKASHI

> 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
>