[RFC PATCH] ARM: Support allocating crashkernel above 4G for LPAE

[Jinjie Ruan]

On 2024/8/6 16:11, Baoquan He wrote:
> On 08/06/24 at 10:46am, Jinjie Ruan wrote:
>>
>>
>> On 2024/8/5 10:56, Baoquan He wrote:
>>> On 08/05/24 at 09:23am, Jinjie Ruan wrote:
>>>>
>>>>
>>>> On 2024/8/2 19:01, Russell King (Oracle) wrote:
>>>>> On Fri, Aug 02, 2024 at 05:25:10PM +0800, Jinjie Ruan wrote:
>>>>>> As ARM LPAE feature support accessing memory beyond the 4G limit, define
>>>>>> HAVE_ARCH_CRASHKERNEL_RESERVATION_HIGH macro to support reserving crash
>>>>>> memory above 4G for ARM32 LPAE.
>>>>>>
>>>>>> No test because there is no LPAE ARM32 hardware.
>>>>>
>>>>> Why are you submitting patches for features you can't test?
>>>>>
>>>>> I'm not going to apply this without it being properly tested, because I
>>>>> don't believe that this will work in the generic case.
>>>>>
>>>>> If the crash kernel is located in memory outside of the lower 4GiB of
>>>>> address space, and there is no alias within physical address space
>>>>> for that memory, then there is *no* *way* for such a kernel to boot.
>>>>
>>>> I'm sorry that I released this patch without testing it. I actually
>>>> intended to bring up this issue for discussion. If anyone has the
>>>> environment to test it, that would be great. In the meantime, we could
>>>> have a discussion on the significance and relevance of this approach.
>>>
>>> I don't know arm32 and its LPAE. I know a little about x86_32 where
>>> crashkernel can only be reserved below 896M because of the virtual
>>> memory layout, and all memory above that is high memory which can't be
>>> used as kernel memory directly. So from this patch, arm32 is different
>>> than x86_32.
>>
>> Hi，Baoquan
>>
>> Does the following code make sense? Now parse_crashkernel() use
>> HAVE_ARCH_CRASHKERNEL_RESERVATION_HIGH macro to parse "high", but use
>> CONFIG_64BIT when reserving "low" memory in reserve_crashkernel_low().
> 
> I am fine with it. BUT have you addressed Russell's concern, e.g how to
> test it actually?

Thank you! Let me find the test environment.

> 
>>
>> And when LPAE is enabled in ARM32, and "high" is reserved,
>> reserve_crashkernel_low() need also function ok.
>>
>> --- a/kernel/crash_reserve.c
>> +++ b/kernel/crash_reserve.c
>> @@ -354,7 +354,7 @@ early_param("crashkernel", parse_crashkernel_dummy);
>>  #ifdef CONFIG_ARCH_HAS_GENERIC_CRASHKERNEL_RESERVATION
>>  static int __init reserve_crashkernel_low(unsigned long long low_size)
>>  {
>> -#ifdef CONFIG_64BIT
>> +#ifdef HAVE_ARCH_CRASHKERNEL_RESERVATION_HIGH
>>         unsigned long long low_base;
>>
>>
>>>
>>>>
>>>>>
>>>>> So, right now I believe this patch to be *fundamentally* wrong.
>>>>>
>>>>
>>>
>>>
>>
> 
>