On Thu, Feb 18, 2016 at 10:20:24PM +0530, Aneesh Kumar K.V wrote:

Hello,

This is a large series, mostly consisting of code movement. No new features
are done in this series. The changes are done to accomodate the upcoming new memory
model in future powerpc chips. The details of the new MMU model can be found at

 http://ibm.biz/power-isa3 (Needs registration). I am including a summary of the changes below.

This doesn't apply against Linus' current tree - have you already
posted the prerequisite patches?  If so, what's the subject of the
0/N patch of the prerequisite series?

ISA 3.0 adds support for the radix tree style of MMU with full
virtualization and related control mechanisms that manage its
coexistence with the HPT. Radix-using operating systems will
manage their own translation tables instead of relying on hcalls.

Radix style MMU model requires us to do a 4 level page table
with 64K and 4K page size. The table index size different page size
is listed below

PGD -> 13 bits
PUD -> 9 (1G hugepage)
PMD -> 9 (2M huge page)
PTE -> 5 (for 64k), 9 (for 4k)

We also require the page table to be in big endian format.

The changes proposed in this series enables us to support both
hash page table and radix tree style MMU using a single kernel
with limited impact. The idea is to change core page table
accessors to static inline functions and later hotpatch them
to switch to hash or radix tree functions. For ex:

static inline int pte_write(pte_t pte)
{
       if (radix_enabled())
               return rpte_write(pte);
        return hlpte_write(pte);
}

Given that with a hash-based MMU, the Linux page tables are purely a
software construct, I don't see why this complexity is necessary.  We
can make the PTE have the same format on radix and hash instead.  I
have a patch series that does that almost ready to post.

On boot we will hotpatch the code so as to avoid conditional operation.

The other two major change propsed in this series is to switch hash
linux page table to a 4 level table in big endian format. This is
done so that functions like pte_val(), pud_populate() doesn't need
hotpatching and thereby helps in limiting runtime impact of the changes.

Right, I agree with this.

Paul.

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