[RFC PATCH 0/2] arm64: Add support for 48-bit Physical Addresses

[Radha Mohan]

On Wed, Nov 27, 2013 at 11:29 PM, Marc Zyngier <marc.zyngier at arm.com> wrote:
> On 27/11/13 15:33, Radha Mohan wrote:
>> On Wed, Nov 27, 2013 at 4:34 PM, Marc Zyngier <marc.zyngier at arm.com> wrote:
>>> [CC-ing the maintainers - seems odd they were not cc-ed the first place]
>>>
>>> On 27/11/13 07:34, mohun106 at gmail.com wrote:
>>>> From: Radha Mohan Chintakuntla <rchintakuntla at cavium.com>
>>>>
>>>> This patch series provides an implementation of supporting 48-bit
>>>> Physical Addresses for ARMv8 platforms. It is the maximum width that
>>>> any ARMv8 based processor can support.
>>>>
>>>> The implementation extends the existing support of 40-bit PA.The kernel
>>>> and user space will now be able to access 128TB each. With 4KB page size
>>>> the Linux now will be using 4 levels of page tables by making use of
>>>> 'pud'. And with 64KB page size the Linux will be using 3 levels of page
>>>> tables.
>>>>
>>>> The code has been tested with LTP.
>>>
>>> Aside from finding out whether or not this is a useful change, this
>>> breaks KVM, more specifically the way kernel pages are mapped into HYP.
>>> Also, guests will still be limited to 40-bit IPAs, and the stage-2
>>> output range needs to be addressed as well.
>>
>> This is useful for platforms (like ours) that will use 48-bit PAs.
>> Sooner or later there will
>> be such processors.
>
> So should the 4-level page table cost be unconditionally forced onto all
> implementations?
>

We can have a kernel compile-time config option so that platforms having
a 48-bit PA can select that option. But having it will create the code something
like below

#ifndef CONFIG_ARM64_64KB_PAGES
#ifdef CONFIG_ARCH_SUPPORTS_48BIT_PA
...
..
#else
..
..
#endif
#else /* !CONFIG_ARM64_64KB_PAGES */
#ifdef CONFIG_ARCH_SUPPORTS_48BIT_PA
..
..
#else
..
..
#endif
#endif /* CONFIG_ARM64_64KB_PAGES */

And we might need to do this at lot of places. Is this ok?

>> And yes, I haven't checked on KVM.
>
> Yeah, tiny detail... ;-)
>
>         M.
> --
> Jazz is not dead. It just smells funny...
>