Re: [RFC v5 02/38] powerpc: Free up four 64K PTE bits in 64K backed HPTE pages
From: Balbir Singh <bsingharora@gmail.com>
Date: 2017-07-12 03:11:08
Also in:
linux-arch, linux-mm, lkml
On Tue, 11 Jul 2017 08:44:15 -0700 Ram Pai [off-list ref] wrote:
On Tue, Jul 11, 2017 at 03:59:59PM +1000, Balbir Singh wrote:quoted
On Wed, 5 Jul 2017 14:21:39 -0700 Ram Pai [off-list ref] wrote:quoted
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. Balbir Singh.