Re: [PATCH v2 3/3] powerpc/mm: Speed up computation of base and actual page size for a HPTE

On Wed, Sep 07, 2016 at 04:17:09PM +1000, Paul Mackerras wrote:

This replaces a 2-D search through an array with a simple 8-bit table
lookup for determining the actual and/or base page size for a HPT entry.

The encoding in the second doubleword of the HPTE is designed to encode
the actual and base page sizes without using any more bits than would be
needed for a 4k page number, by using between 1 and 8 low-order bits of
the RPN (real page number) field to encode the page sizes.  A single
"large page" bit in the first doubleword indicates that these low-order
bits are to be interpreted like this.

We can determine the page sizes by using the low-order 8 bits of the RPN
to look up a 256-entry table.  For actual page sizes less than 1MB, some
of the upper bits of these 8 bits are going to be real address bits, but
we can cope with that by replicating the entries for those smaller page
sizes.

While we're at it, let's move the hpte_page_size() and hpte_base_page_size()
functions from a KVM-specific header to a header for 64-bit HPT systems,
since this computation doesn't have anything specifically to do with KVM.

Signed-off-by: Paul Mackerras <redacted>
---
v2: added more comments as suggested by Aneesh

 arch/powerpc/include/asm/book3s/64/mmu-hash.h | 37 ++++++++++++
 arch/powerpc/include/asm/kvm_book3s_64.h      | 87 +++------------------------
 arch/powerpc/include/asm/mmu.h                |  1 +
 arch/powerpc/mm/hash_native_64.c              | 42 +------------
 arch/powerpc/mm/hash_utils_64.c               | 55 +++++++++++++++++

This of course touches two maintainers' areas.  Michael and Paolo, how
do you want to proceed here?  Can this just go through Michael's tree?
Or should I make a topic branch off Linus' tree that you can both
pull, or should I split the patch into two (i.e. everything except the
kvm_book3s_64.h change in the first patch, and the kvm_book3s_64.h
change in the second) and get Michael to put the first one in a topic
branch that I can then pull and apply the second patch onto?

Thanks,
Paul.