Re: [PATCH v12 1/5] efi: ARM/arm64: ignore DT memory nodes instead of removing them

[Ard Biesheuvel]

On 23 February 2016 at 13:16, Will Deacon <will.deacon@xxxxxxx> wrote:
> On Tue, Feb 23, 2016 at 11:58:05AM +0000, Mark Rutland wrote:
>> On Mon, Feb 22, 2016 at 05:58:19PM -0800, David Daney wrote:
>> > From: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
>> >
>> > There are two problems with the UEFI stub DT memory node removal
>> > routine:
>> > - it deletes nodes as it traverses the tree, which happens to work
>> >   but is not supported, as deletion invalidates the node iterator;
>> > - deleting memory nodes entirely may discard annotations in the form
>> >   of additional properties on the nodes.
>> >
>> > Since the discovery of DT memory nodes occurs strictly before the
>> > UEFI init sequence, we can simply clear the memblock memory table
>> > before parsing the UEFI memory map. This way, it is no longer
>> > necessary to remove the nodes, so we can remove that logic from the
>> > stub as well.
>>
>> This is a little bit scary, but I guess this works.
>>
>> My only concern is that when we get kexec, a subsequent kernel must also
>> have EFI memory map support, or things go bad for the next EFI-aware
>> kernel after that (as things like the runtime services may have been
>> corrupted by the kernel in the middle). It's difficult to fix the
>> general case later.
>>
>> A different option would be to support status="disabled" for the memory
>> nodes, and ignore these in early_init_dt_scan_memory. That way a kernel
>> cannot use memory without first having parsed the EFI memory map, and we
>> can still get NUMA info from the disabled nodes.
>
> So in that case, the middle, non-EFI kernel would fail to boot?
> Realistically, once you've kexec'd a non-EFI payload, I don't think you
> can rely on the EFI state remaining intact for future EFI applications.
>
> Is this really something we should be trying to police in the kernel?
>

Well, we could add entries to /reserved-memory in the stub for all the
regions UEFI cares about, that would probably be sufficient to fix
this case.