Re: [PATCH 0/4] support reserving crashkernel above 4G on arm64 kdump

[Chen Zhou <chenzhou10@huawei.com>]

On 2019/6/6 0:32, James Morse wrote:
> Hi!
> 
> On 07/05/2019 04:50, Chen Zhou wrote:
>> We use crashkernel=X to reserve crashkernel below 4G, which will fail
>> when there is no enough memory. Currently, crashkernel=Y@X can be used
>> to reserve crashkernel above 4G, in this case, if swiotlb or DMA buffers
>> are requierd, capture kernel will boot failure because of no low memory.
> 
>> When crashkernel is reserved above 4G in memory, kernel should reserve
>> some amount of low memory for swiotlb and some DMA buffers. So there may
>> be two crash kernel regions, one is below 4G, the other is above 4G.
> 
> This is a good argument for supporting the 'crashkernel=...,low' version.
> What is the 'crashkernel=...,high' version for?
> 
> Wouldn't it be simpler to relax the ARCH_LOW_ADDRESS_LIMIT if we see 'crashkernel=...,low'
> in the kernel cmdline?
> 
> I don't see what the 'crashkernel=...,high' variant is giving us, it just complicates the
> flow of reserve_crashkernel().
> 
> If we called reserve_crashkernel_low() at the beginning of reserve_crashkernel() we could
> use crashk_low_res.end to change some limit variable from ARCH_LOW_ADDRESS_LIMIT to
> memblock_end_of_DRAM().
> I think this is a simpler change that gives you what you want.

According to your suggestions, we should do like this:
1. call reserve_crashkernel_low() at the beginning of reserve_crashkernel()
2. mark the low region as 'nomap'
3. use crashk_low_res.end to change some limit variable from ARCH_LOW_ADDRESS_LIMIT to
memblock_end_of_DRAM()
4. rename crashk_low_res as "Crash kernel (low)" for arm64
5. add an 'linux,low-memory-range' node in DT

Do i understand correctly?

> 
> 
>> Then
>> Crash dump kernel reads more than one crash kernel regions via a dtb
>> property under node /chosen,
>> linux,usable-memory-range = <BASE1 SIZE1 [BASE2 SIZE2]>.
> 
> Won't this break if your kdump kernel doesn't know what the extra parameters are?
> Or if it expects two ranges, but only gets one? These DT properties should be treated as
> ABI between kernel versions, we can't really change it like this.
> 
> I think the 'low' region is an optional-extra, that is never mapped by the first kernel. I
> think the simplest thing to do is to add an 'linux,low-memory-range' that we
> memblock_add() after memblock_cap_memory_range() has been called.
> If its missing, or the new kernel doesn't know what its for, everything keeps working.
> 
> 
>> Besides, we need to modify kexec-tools:
>>   arm64: support more than one crash kernel regions(see [1])
> 
>> I post this patch series about one month ago. The previous changes and
>> discussions can be retrived from:
> 
> Ah, this wasn't obvious as you've stopped numbering the series. Please label the next one
> 'v6' so that we can describe this as 'v5'. (duplicate numbering would be even more confusing!)
> 
ok.

> 
> Thanks,
> 
> James
> 
> .
> 

Thanks,
Chen Zhou