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SubjectRe: [RFC v5 02/38] powerpc: Free up four 64K PTE bits in 64K backed HPTE pages
On Wed, Jul 12, 2017 at 01:10:51PM +1000, Balbir Singh wrote:
> On Tue, 11 Jul 2017 08:44:15 -0700
> Ram Pai <linuxram@us.ibm.com> wrote:
>
> > On Tue, Jul 11, 2017 at 03:59:59PM +1000, Balbir Singh wrote:
> > > On Wed, 5 Jul 2017 14:21:39 -0700
> > > Ram Pai <linuxram@us.ibm.com> wrote:
> > >
> > > > Rearrange 64K PTE bits to free up bits 3, 4, 5 and 6
> > > > in the 64K backed HPTE pages. This along with the earlier
> > > > patch will entirely free up the four bits from 64K PTE.
> > > > The bit numbers are big-endian as defined in the ISA3.0
> > > >
> > > > This patch does the following change to 64K PTE backed
> > > > by 64K HPTE.
> > > >
> > > > H_PAGE_F_SECOND (S) which occupied bit 4 moves to the
> > > > second part of the pte to bit 60.
> > > > H_PAGE_F_GIX (G,I,X) which occupied bit 5, 6 and 7 also
> > > > moves to the second part of the pte to bit 61,
> > > > 62, 63, 64 respectively
> > > >
> > > > since bit 7 is now freed up, we move H_PAGE_BUSY (B) from
> > > > bit 9 to bit 7.
> > > >
> > > > The second part of the PTE will hold
> > > > (H_PAGE_F_SECOND|H_PAGE_F_GIX) at bit 60,61,62,63.
> > > >
> > > > Before the patch, the 64K HPTE 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| |S |G |I |X |x|B|x|x|x|................|.|.|.|.| <- primary pte
> > > > '_'_'_'_'__'__'__'__'_'_'_'_'_'________________'_'_'_'_'
> > > > | | | | | | | | | | | | |..................| | | | | <- secondary pte
> > > > '_'_'_'_'__'__'__'__'_'_'_'_'__________________'_'_'_'_'
> > > >
> > >
> > > It's not entirely clear what the secondary pte contains
> > > today and how many of the bits are free today?
> >
> > The secondary pte today is not used for anything for 64k-hpte
> > backed ptes. It gets used the moment the pte gets backed by
> > 4-k hptes. Till then the bits are available. And this patch
> > makes use of that knowledge.
>
> OK.. but does this mean subpage-protection? Or do you mean
> page size demotion? I presume it's the later.

Yes. the later.
RP

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