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

[mark.rutland@arm.com (Mark Rutland)]

On Wed, Jan 20, 2016 at 03:59:08PM +0100, 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)
> 
> 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

Ok. I had assumed that getting the BE kernel to deal with the UEFI
memory map would be a bit more involved.

I'm happy to be proven wrong. :)

Thanks,
Mark.