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 later. +#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 */