[PATCH 2/2] powerpc/fadump: fix additional param memory reservation for HASH MMU

[Hari Bathini]

On 23/01/25 7:54 pm, Avnish Chouhan wrote:
> On 2025-01-23 15:26, Hari Bathini wrote:
>> On 20/01/25 11:05 pm, Sourabh Jain wrote:
>>> Commit 683eab94da75bc ("powerpc/fadump: setup additional parameters for
>>> dump capture kernel") introduced the additional parameter feature in
>>> fadump for HASH MMU with the understanding that GRUB does not use the
>>> memory area between 640MB and 768MB for its operation.
>>>
>>> However, the patch ("powerpc: increase MIN RMA size for CAS
>>> negotiation") changes the MIN RMA size to 768MB, allowing GRUB to use
>>> memory up to 768MB. This makes the fadump reservation for the additional
>>> parameter feature for HASH MMU unreliable.
>>>
>>> To address this, adjust the memory range for the additional parameter in
>>> fadump for HASH MMU. This will ensure that GRUB does not overwrite the
>>> memory reserved for fadump's additional parameter in HASH MMU.
>>>
>>
>>> The new policy for the memory range for the additional parameter in HASH
>>> MMU is that the first memory block must be larger than the MIN_RMA size,
>>> as the bootloader can use memory up to the MIN_RMA size. The range
>>> should be between MIN_RMA and the RMA size (ppc64_rma_size), and it must
>>> not overlap with the fadump reserved area.
>>
>> IIRC, even memory above MIN_RMA is used by the bootloader except for
>> 640MB to 768MB (assuming RMA size is >768MB). So, how does this change
>> guarantee that the bootloader is not using memory reserved for bootargs?
>>
>> Avnish, earlier, bootloader was using RUNTIME_MIN_SPACE (128MB) starting
>> top-down at 768MB earlier. With MIN_RMA changed to 768MB, is bootloader
>> still using the concept of RUNTIME_MIN_SPACE to set aside some memory
>> for kernel to use. If yes, where exactly is it allocating this space
>> now? Also, rtas instantiates top-down at 768MB. Would that not have
>> a conflict with grub allocations without RUNTIME_MIN_SPACE at 768MB?
>>
>> - Hari
> 
> Hi Hari,

Hi Avnish,

> The RUNTIME_MIN_SPACE is the space left aside by Grub is within the 
> MIN_RMA size. Grub won't use memory beyond the MIN_RMA. With this 
> change, we haven't changed the RUNTIME_MIN_SPACE behavior. Grub will 
> still keep the 128 MB space in MIN_RMA for loading stock kernel and initrd.

IIUC, you mean, 640MB to 768MB is not used by Grub even if MIN_RMA
is at 768MB? If that is true, this change is not needed, as fadump
could still use the memory between 640MB to 768MB, right?
Am I missing something here..

- Hari