Re: [RFC v5 02/38] powerpc: Free up four 64K PTE bits in 64K backed HPTE pages
From: Ram Pai
Date: Tue Jul 11 2017 - 11:44:58 EST
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@xxxxxxxxxx> 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.
Will add some words in the patch description towards this.
> Balbir Singh.