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Date:   Tue, 1 Sep 2020 18:13:07 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     chenzhou <chenzhou10@huawei.com>
Cc:     Dave Young <dyoung@redhat.com>, will@kernel.org,
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        guohanjun@huawei.com, xiexiuqi@huawei.com, huawei.libin@huawei.com,
        wangkefeng.wang@huawei.com
Subject: Re: [PATCH v11 5/5] kdump: update Documentation about crashkernel
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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