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

[Avnish Chouhan <avnish@linux.ibm.com>]

On 2025-01-31 20:44, Hari Bathini wrote:
> 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,

No. As we are changing MIN_RMA to 768 MB, GRUB can use memory till 768 
MB if required.

Regards,
Avnish Chouhan

> 
> - Hari