Re: [RFC v2 01/12] powerpc: Free up four 64K PTE bits in 4K backed hpte pages.

From: Anshuman Khandual
Date: Thu Jun 22 2017 - 05:07:57 EST

On 06/17/2017 09:22 AM, Ram Pai wrote:
> 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 patch does the following change to the 64K PTE format
> H_PAGE_BUSY moves from bit 3 to bit 9
> H_PAGE_F_SECOND which occupied bit 4 moves to the second part
> of the pte.
> H_PAGE_F_GIX which occupied bit 5, 6 and 7 also moves to the
> second part of the pte.
> the four bits((H_PAGE_F_SECOND|H_PAGE_F_GIX) that represent a slot
> is initialized to 0xF indicating an invalid slot. If a hpte
> gets cached in a 0xF slot(i.e 7th slot of secondary), it is
> released immediately. In other words, even though 0xF is a
> valid slot we discard and consider it as an invalid
> slot;i.e hpte_soft_invalid(). This gives us an opportunity to not
> depend on a bit in the primary PTE in order to determine the
> validity of a slot.
> When we release a hpte in the 0xF slot we also release a
> legitimate primary slot and unmap that entry. This is to
> ensure that we do get a legimate non-0xF slot the next time we
> retry for a slot.
> Though treating 0xF slot as invalid reduces the number of available
> slots and may have an effect on the performance, the probabilty
> of hitting a 0xF 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.
> This idea was jointly developed by Paul Mackerras, Aneesh, Michael
> Ellermen and myself.
> 4K PTE format remain unchanged currently.

Scanned through the PTE format again for hash 64K and 4K. It seems
to me that there might be 5 free bits already present on the PTE
format. I might have seriously mistaken something here :) Please
correct me if that is not the case. _RPAGE_RPN* I think is applicable
only for hash page table format and will not be available for radix

+#define _PAGE_FREE_1 0x0000000000000040UL /* Not used */
+#define _RPAGE_SW0 0x2000000000000000UL /* Not used */
+#define _RPAGE_SW1 0x0000000000000800UL /* Not used */
+#define _RPAGE_RPN42 0x0040000000000000UL /* Not used */
+#define _RPAGE_RPN41 0x0020000000000000UL /* Not used */