[PATCH 1/6] powerpc: Free up four 64K PTE bits in 4K backed HPTE pages
Ram Pai
linuxram at us.ibm.com
Thu Jul 27 02:06:51 AEST 2017
On Wed, Jul 26, 2017 at 04:05:48PM +0530, Aneesh Kumar K.V wrote:
> Ram Pai <linuxram at 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 at linux.vnet.ibm.com>
> > Signed-off-by: Ram Pai <linuxram at 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
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