[PATCH V2 09/11] powerpc/mm: Lower the max real address to 51 bits

[Aneesh Kumar K.V]

On Friday 17 March 2017 02:56 AM, Benjamin Herrenschmidt wrote:
> On Thu, 2017-03-16 at 16:02 +0530, Aneesh Kumar K.V wrote:
>> Max value supported by hardware is 51 bits address. Radix page table define
>> a slot of 57 bits for future expansion. We restrict the value supported in
>> linux kernel 51 bits, so that we can use the bits between 57-51 for storing
>> hash linux page table bits. This is done in the next patch.
>
> All of them ? I would keep some for future backward compatibility. It's likely
> that a successor to P9 will have more physical address bits. I feel nervous
> limiting to precisely what P9 supports.
>

What do you want to keep as MAX PFN bits here ?. Any new expansion will 
eat into the software bits defined by Radix and hence can't be used
for generic features.


>> This will free up the software page table bits to be used for features
>> that are needed for both hash and radix. The current hash linux page table
>> format doesn't have any free software bits. Moving hash linux page table
>> specific bits to top of RPN field free up the software bits for other purpose.
>>

-aneesh