Re: [PATCH 1/6] powerpc: Free up four 64K PTE bits in 4K backed HPTE pages

[Ram Pai <linuxram@us.ibm.com>]

On Wed, Jul 26, 2017 at 04:05:48PM +0530, Aneesh Kumar K.V wrote:
> Ram Pai <linuxram@us.ibm.com> writes:
> 
> > Rearrange 64K PTE bits to  free  up  bits 3, 4, 5  and  6,
> > in the 4K backed HPTE pages.These bits continue to be used
> > for 64K backed HPTE pages in this patch, but will be freed
> > up in the next patch. The  bit  numbers are big-endian  as
> > defined in the ISA3.0
> >
> > The patch does the following change to the 4k htpe backed
> > 64K PTE's format.
> >
> > H_PAGE_BUSY moves from bit 3 to bit 9 (B bit in the figure
> > 		below)
> > V0 which occupied bit 4 is not used anymore.
> > V1 which occupied bit 5 is not used anymore.
> > V2 which occupied bit 6 is not used anymore.
> > V3 which occupied bit 7 is not used anymore.
> >
> > Before the patch, the 4k backed 64k PTE format was as follows
> >
> >  0 1 2 3 4  5  6  7  8 9 10...........................63
> >  : : : : :  :  :  :  : : :                            :
> >  v v v v v  v  v  v  v v v                            v
> >
> > ,-,-,-,-,--,--,--,--,-,-,-,-,-,------------------,-,-,-,
> > |x|x|x|B|V0|V1|V2|V3|x| | |x|x|................|x|x|x|x| <- primary pte
> > '_'_'_'_'__'__'__'__'_'_'_'_'_'________________'_'_'_'_'
> > |S|G|I|X|S |G |I |X |S|G|I|X|..................|S|G|I|X| <- secondary pte
> > '_'_'_'_'__'__'__'__'_'_'_'_'__________________'_'_'_'_'
> >
> > After the patch, the 4k backed 64k PTE format is as follows
> >
> >  0 1 2 3 4  5  6  7  8 9 10...........................63
> >  : : : : :  :  :  :  : : :                            :
> >  v v v v v  v  v  v  v v v                            v
> >
> > ,-,-,-,-,--,--,--,--,-,-,-,-,-,------------------,-,-,-,
> > |x|x|x| |  |  |  |  |x|B| |x|x|................|.|.|.|.| <- primary pte
> > '_'_'_'_'__'__'__'__'_'_'_'_'_'________________'_'_'_'_'
> > |S|G|I|X|S |G |I |X |S|G|I|X|..................|S|G|I|X| <- secondary pte
> > '_'_'_'_'__'__'__'__'_'_'_'_'__________________'_'_'_'_'
> >
> > the four  bits S,G,I,X (one quadruplet per 4k HPTE) that
> > cache  the  hash-bucket  slot  value, is initialized  to
> > 1,1,1,1 indicating -- an invalid slot.   If  a HPTE gets
> > cached in a 1111  slot(i.e 7th  slot  of  secondary hash
> > bucket), it is  released  immediately. In  other  words,
> > even  though 1111   is   a valid slot  value in the hash
> > bucket, we consider it invalid and  release the slot and
> > the HPTE.  This  gives  us  the opportunity to determine
> > the validity of S,G,I,X  bits  based on its contents and
> > not on any of the bits V0,V1,V2 or V3 in the primary PTE
> >
> > When   we  release  a    HPTE    cached in the 1111 slot
> > we also    release  a  legitimate   slot  in the primary
> > hash bucket  and  unmap  its  corresponding  HPTE.  This
> > is  to  ensure   that  we do get a HPTE cached in a slot
> > of the primary hash bucket, the next time we retry.
> >
> > Though  treating  1111  slot  as  invalid,  reduces  the
> > number of  available  slots  in the hash bucket and  may
> > have  an  effect   on the performance, the probabilty of
> > hitting a 1111 slot is extermely low.
> >
> > Compared  to  the  current   scheme, the above described
> > scheme  reduces  the  number of false hash table updates
> > significantly   and    has  the   added   advantage   of
> > releasing  four  valuable  PTE bits for other purpose.
> >
> > NOTE:even though bits 3, 4, 5, 6, 7 are  not  used  when
> > the  64K  PTE is backed by 4k HPTE,  they continue to be
> > used  if  the  PTE  gets  backed  by 64k HPTE.  The next
> > patch will decouple that aswell, and truely  release the
> > bits.
> >
> > This idea was jointly developed by Paul Mackerras,
> > Aneesh, Michael Ellermen and myself.
> >
> > 4K PTE format remains unchanged currently.
> >
> > The patch does the following code changes
> > a) PTE flags are split between 64k and 4k  header files.
> > b) __hash_page_4K()  is  reimplemented   to reflect the
> >    above logic.
> >
> > Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> > Signed-off-by: Ram Pai <linuxram@us.ibm.com>
> > ---
> >  arch/powerpc/include/asm/book3s/64/hash-4k.h  |    2 +
> >  arch/powerpc/include/asm/book3s/64/hash-64k.h |    8 +--
> >  arch/powerpc/include/asm/book3s/64/hash.h     |    1 -
> >  arch/powerpc/mm/hash64_64k.c                  |   74 ++++++++++++++++---------
> >  arch/powerpc/mm/hash_utils_64.c               |    4 +-
> >  5 files changed, 55 insertions(+), 34 deletions(-)
> >
> > diff --git a/arch/powerpc/include/asm/book3s/64/hash-4k.h b/arch/powerpc/include/asm/book3s/64/hash-4k.h
> > index 0c4e470..f959c00 100644
> > --- a/arch/powerpc/include/asm/book3s/64/hash-4k.h
> > +++ b/arch/powerpc/include/asm/book3s/64/hash-4k.h
> > @@ -16,6 +16,8 @@
> >  #define H_PUD_TABLE_SIZE	(sizeof(pud_t) << H_PUD_INDEX_SIZE)
> >  #define H_PGD_TABLE_SIZE	(sizeof(pgd_t) << H_PGD_INDEX_SIZE)
> >
> > +#define H_PAGE_BUSY	_RPAGE_RSV1     /* software: PTE & hash are busy */
> > +
> >  /* PTE flags to conserve for HPTE identification */
> >  #define _PAGE_HPTEFLAGS (H_PAGE_BUSY | H_PAGE_HASHPTE | \
> >  			 H_PAGE_F_SECOND | H_PAGE_F_GIX)
> > diff --git a/arch/powerpc/include/asm/book3s/64/hash-64k.h b/arch/powerpc/include/asm/book3s/64/hash-64k.h
> > index 9732837..62e580c 100644
> > --- a/arch/powerpc/include/asm/book3s/64/hash-64k.h
> > +++ b/arch/powerpc/include/asm/book3s/64/hash-64k.h
> > @@ -12,18 +12,14 @@
> >   */
> >  #define H_PAGE_COMBO	_RPAGE_RPN0 /* this is a combo 4k page */
> >  #define H_PAGE_4K_PFN	_RPAGE_RPN1 /* PFN is for a single 4k page */
> > +#define H_PAGE_BUSY	_RPAGE_RPN42     /* software: PTE & hash are busy */
> 
> 
> Why are we moving H_PAGE_BUSY. Right now 4k and 64k linux page table
> format looks similar.

The goal is to clear off all the _RPAGE_RSV* bits so that they can be
used for protection keys.  the aim is to keep the protection-bits in the
_RPAGE_RSV* bits, so that they will work as-is whenever radix MMU enables
protection keys.

Yes this makes the PTE format differ from 4k PTE. Hopefully it is a
small inconvenience. The PTE format for 4K is anyway not exactly the
same compared to 64K PTE format. For example, higher RPN bits are 
used on 4K but not on 64k. lower RPN bits are used on 64k but not
on 4k.

RP
> We use the lower RPN bits only for subpage
> tracking/details.
> 
> 
> > +
> >  /*
> >   * We need to differentiate between explicit huge page and THP huge
> >   * page, since THP huge page also need to track real subpage details
> >   */
> >  #define H_PAGE_THP_HUGE  H_PAGE_4K_PFN
> >
> 
> 
> -aneesh

-- 
Ram Pai