[PATCH v26 0/7] arm64: add kdump support

[Ard Biesheuvel]

On 16 September 2016 at 17:04, James Morse <james.morse at arm.com> wrote:
> (Cc: Ard),
>
> Mark, Ard, how does/will reserved-memory work on an APCI only system?
>

It works by accident, at the moment. We used to ignore both
/memreserve/s and the /reserved-memory node, but due to some unrelated
refactoring, we ended up honouring the reserved-memory node when
booting via UEFI

I proposed some patches a while ago to at least check the
reservations, given that UEFI itself is unaware of them and may end up
occupying a region that should have been reserved.

http://article.gmane.org/gmane.linux.kernel.efi/6464

>
> On 07/09/16 05:29, AKASHI Takahiro wrote:
>>     v26-specific note: After a comment from Rob[0], an idea of adding
>>     "linux,usable-memory-range" was dropped. Instead, an existing
>>     "reserved-memory" node will be used to limit usable memory ranges
>>     on crash dump kernel.
>>     This works not only on UEFI/ACPI systems but also on DT-only systems,
>>     but if he really insists on using DT-specific "usable-memory" property,
>>     I will post additional patches for kexec-tools. Those would be
>>     redundant, though.
>>     Even in that case, the kernel will not have to be changed.
>
> Some narrative on how the old memory ranges get reserved, as there is no longer
> any code in the series doing this, (which is pretty neat!):
>
> kexec-tools parses the list of memory ranges in /proc/iomem, and adds a node to
> the /reserved-memory for System RAM ranges that don't cover the crash kernel.
> Decompiling the crash-kernel DT from Seattle, it looks roughly like this:
>
>         reserved-memory {
>                 ranges;
>                 #size-cells = <0x2>;
>                 #address-cells = <0x2>;
>
>                 crash_dump at 83ffe50000 {
>                         no-map;
>                         reg = <0x83 0xffe50000 0x0 0x1b0000>;
>                 };
>
>                 [ ... ]
>         };
>
>
> 'no-map' means its doing the same thing to memblock as
> 'linux,usable-memory-range' did in earlier versions,
> early_init_dt_reserve_memory_arch() takes no-map to mean memblock_remove().
> We trigger the removing via early_init_fdt_scan_reserved_mem() in
> arch/arm64/mm/init.c. This happens later than before, but its before the
> crashkernel and cma ranges get reserved.
>
> One difference I can see is that before we avoided memblock_remove()ing ranges
> that were also in memblock.nomap. This was to avoid the ACPI tables getting
> mapped as device memory by mistake, this is fixed by [1]. Now these ranges are
> published in /proc/iomem as 'reserved' and won't get covered by a
> reserved-memory node, and so we don't need to check memblock.nomap when
> memblock_remove()ing.
>
>
> The only odd thing I can see is for a (mythical?) pure-ACPI system. The EFI stub
> will create a DT with a chosen node containing pointers to the memory map and
> the efi command line. Now such as system may also grow a /reserved-memory node
> after kdump. I don't think this is a problem, but it may not match how an
> acpi-only system reserves memory. (how does that work?)
>
>
>> [1] "arm64: mark reserved memblock regions explicitly in iomem"
>>     http://lists.infradead.org/pipermail/linux-arm-kernel/2016-August/450433.html
>
> This is queued in Will's arm64/for-next/core,
>
>> [2] "efi: arm64: treat regions with WT/WC set but WB cleared as memory"
>>     http://lists.infradead.org/pipermail/linux-arm-kernel/2016-August/451491.html
>
> This is queued in tip, but I can't see why kdump depends on it. It only has an
> effect if the uefi memory map has !WB regions that linux needs to use.
>
>
>
> Thanks,
>
> James
>