Re: [PATCH v11 5/5] kdump: update Documentation about crashkernel

Catalin Marinas
2020-09-01T17:13:07Z

On Tue, Aug 18, 2020 at 03:07:04PM +0800, chenzhou wrote:
> On 2020/8/10 14:03, Dave Young wrote:
> >>> Previously I remember we talked about to use similar logic as X86, but I
> >>> remember you mentioned on some arm64 platform there could be no low
> >>> memory at all. Is this not a problem now for the fallback? Just be
> >>> curious, thanks for the update, for the common part looks good.
> >>
> >> Did you mean this discuss: https://lkml.org/lkml/2019/12/27/122?
> > I meant about this reply instead :)
> > https://lkml.org/lkml/2020/1/16/616
>
> Sorry for not repley in time, I was on holiday last week.
>
> The platform James mentioned may exist for which have no devices and
> need no low memory.

If there is no memory below 4GB, the arm64 kernel assumes that the
32-bit devices will have some DMA offsets shifting the addresses to the
bottom of the available RAM. So even if RAM starts above 4GB, we
ZONE_DMA32 will be allocated in the bottom 4GB of the high memory (and
if the hardware designers forgot to shift those DMA accesses, we don't
have to support the platform ;)).

So the arm64 notion of low memory differs slightly from the x86 one.

--
Catalin