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

[Ram Pai]

On Thu, Jun 22, 2017 at 02:37:27PM +0530, Anshuman Khandual wrote:
> 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 */
> 

The bits are chosen to future proof for radix implementation.
_RPAGE_SW* will eat into what is available for software in the future,
and these key-bits will certainly be something that the radix
hardware will read, in the future.

The _RPAGE_RPN* bits cannot be relied on for radix.

But finally the bits that we chose (H_PAGE_F_SECOND|H_PAGE_F_GIX) had
the best potential for giving us the highest number of free bits with
relatively less effort.

RP