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

[Mark Rutland]

On Thu, Jan 21, 2016 at 02:43:15PM +0900, AKASHI Takahiro wrote:
> On 01/20/2016 11:59 PM, Ard Biesheuvel wrote:
> >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)
> 
> Just from my curiosity,
> will runtime services be also available on BE kernel with LE uefi?

It may be possible to implement that (I recall that Ard had a go), but
that's far more complicated than simply supporting the EFI memory map,
as you need separate (endian-swapped) page tables and other data
structures, lose the ability to handle exceptions, etc.

All that's suggested above is supporting the memory map.

Thanks,
Mark.