Subject: Re: [PATCH V2 07/29] mm: Make vm_get_page_prot arch specific.

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Mon, 15 Feb 2016 10:10:24 +0530

Paul Mackerras <[email-masked]> writes:

> On Mon, Feb 08, 2016 at 02:50:19PM +0530, Aneesh Kumar K.V wrote:
>> With next generation power processor, we are having a new mmu model
>> [1] that require us to maintain a different linux page table format.
>> 
>> Inorder to support both current and future ppc64 systems with a single
>> kernel we need to make sure kernel can select between different page
>> table format at runtime. With the new MMU (radix MMU) added, we will
>> have to dynamically switch between different protection map. Hence
>> override vm_get_page_prot instead of using arch_vm_get_page_prot. We
>> also drop arch_vm_get_page_prot since only powerpc used it.
>
> What's different about ISA v3.0 that means that the protection_map[]
> entries need to be different?
>
> If it's just different bit assignments for things like _PAGE_READ
> etc., couldn't we fix this up at early boot time by writing new values
> into protection_map[]?  Is there a reason why that wouldn't work, or
> why you don't want to do that?
>

Yes, that is other way to do this. But I thought it is easier to have
different protection_map array for radix and hash. That made the code
more readable.

-aneesh