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

[Jinjie Ruan]

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().

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.
>>>
>>
> 
>