Re: [RFC v3 01/23] powerpc: Free up four 64K PTE bits in 4K backed HPTE pages

[Ram Pai]

On Thu, Jun 22, 2017 at 07:21:03PM +1000, Balbir Singh wrote:
> On Wed, 2017-06-21 at 18:39 -0700, 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  bit  numbers  are big-endian  as
> > defined in the ISA3.0
> > 
> > The patch does the following change to the 64K PTE format
> >
> 
> Why can't we stuff the bits in the VMA and retrieve it from there?
> Basically always get a minor fault in hash and for keys handle
> the fault in do_page_fault() and handle the keys from the VMA?

I think you raise a valid point. We dont necessarily have to program
the pte. the hpte can be programmed directly from the key in the vma.
Just that the code becomes a little ugly to do so, since the
_hash_page_*() functions do not have access to the vma.

However we are also trying to maintain consistency between hpte and rpte
implementation. The keys have to be programmed into the rpte.
The patch is working towards enabling the consistency, so that
the same code can work on both, hpte for now and rpte in the future.

Maybe I can just do what you propose.  However this patch by itself
has value, because it frees up four valuable pte bits, irrespective
of whether we use it for memory keys. Let me see what others have
to say.  

Aneesh: thoughts?

> 
> > 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(). This  gives  us  an opportunity to not
> > depend on a bit in the primary PTE in order to determine the
> > validity of a slot.
> 
> This is not clear, could you please rephrase? What is the bit in the
> primary key we rely on?

(H_PAGE_F_SECOND|H_PAGE_F_GIX) bits, which is big-endian bits 3 4 5 and
6. They are currently used to track the validitiy of the 4k-hptes backing the
64k-pte.   Each bit tracks four 4k-hptes, for a total of sixteen
4k-hptes.


> 
> > 
> > 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.
> >
> 
> It would be helpful if you had a text diagram explaining the PTE bits
> before and after.

ok. will add it in the next version.

> 
> > 4K PTE format remain unchanged currently.
> >
> 
> The code seems to be doing a lot more than the changelog suggests. A few
> functions are completely removed, common code between 64K and 4K has been
> split under #ifndef. It would be good to call all of these out.

ok. will do.

> 
> > Signed-off-by: Ram Pai <linuxram@xxxxxxxxxx>
> > 
> > Conflicts:
> > 	arch/powerpc/include/asm/book3s/64/hash.h
> > ---
> >  arch/powerpc/include/asm/book3s/64/hash-4k.h  |  7 +++
> >  arch/powerpc/include/asm/book3s/64/hash-64k.h | 17 ++++---
> >  arch/powerpc/include/asm/book3s/64/hash.h     | 12 +++--
> >  arch/powerpc/mm/hash64_64k.c                  | 70 +++++++++++++++------------
> >  arch/powerpc/mm/hash_utils_64.c               |  4 +-
> >  5 files changed, 66 insertions(+), 44 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/book3s/64/hash-4k.h b/arch/powerpc/include/asm/book3s/64/hash-4k.h
> > index b4b5e6b..9c2c8f1 100644
> > --- a/arch/powerpc/include/asm/book3s/64/hash-4k.h
> > +++ b/arch/powerpc/include/asm/book3s/64/hash-4k.h
> > @@ -16,6 +16,13 @@
> >  #define H_PUD_TABLE_SIZE	(sizeof(pud_t) << H_PUD_INDEX_SIZE)
> >  #define H_PGD_TABLE_SIZE	(sizeof(pgd_t) << H_PGD_INDEX_SIZE)
> >  
> > +#define H_PAGE_F_SECOND        _RPAGE_RSV2     /* HPTE is in 2ndary HPTEG */
> > +#define H_PAGE_F_GIX           (_RPAGE_RSV3 | _RPAGE_RSV4 | _RPAGE_RPN44)
> > +#define H_PAGE_F_GIX_SHIFT     56
> > +
> > +#define H_PAGE_BUSY	_RPAGE_RSV1     /* software: PTE & hash are busy */
> > +#define H_PAGE_HASHPTE	_RPAGE_RPN43    /* PTE has associated HPTE */
> > +
> >  /* PTE flags to conserve for HPTE identification */
> >  #define _PAGE_HPTEFLAGS (H_PAGE_BUSY | H_PAGE_HASHPTE | \
> >  			 H_PAGE_F_SECOND | H_PAGE_F_GIX)
> > diff --git a/arch/powerpc/include/asm/book3s/64/hash-64k.h b/arch/powerpc/include/asm/book3s/64/hash-64k.h
> > index 9732837..3f49941 100644
> > --- a/arch/powerpc/include/asm/book3s/64/hash-64k.h
> > +++ b/arch/powerpc/include/asm/book3s/64/hash-64k.h
> > @@ -10,20 +10,21 @@
> >   * 64k aligned address free up few of the lower bits of RPN for us
> >   * We steal that here. For more deatils look at pte_pfn/pfn_pte()
> >   */
> > -#define H_PAGE_COMBO	_RPAGE_RPN0 /* this is a combo 4k page */
> > -#define H_PAGE_4K_PFN	_RPAGE_RPN1 /* PFN is for a single 4k page */
> > +#define H_PAGE_COMBO   _RPAGE_RPN0 /* this is a combo 4k page */
> > +#define H_PAGE_4K_PFN  _RPAGE_RPN1 /* PFN is for a single 4k page */
> 
> It would be good to split out these as cleanups, I can't see anything
> change above, its a little confusing to review it.

Will have to split the patch further into smaller chunks. :-)   Will do.

> 
> > +#define H_PAGE_F_SECOND	_RPAGE_RSV2	/* HPTE is in 2ndary HPTEG */
> > +#define H_PAGE_F_GIX	(_RPAGE_RSV3 | _RPAGE_RSV4 | _RPAGE_RPN44)
> > +#define H_PAGE_F_GIX_SHIFT	56
> > +
> > +#define H_PAGE_BUSY	_RPAGE_RPN42     /* software: PTE & hash are busy */
> > +#define H_PAGE_HASHPTE	_RPAGE_RPN43    /* PTE has associated HPTE */
> > +
> >  /*
> >   * We need to differentiate between explicit huge page and THP huge
> >   * page, since THP huge page also need to track real subpage details
> >   */
> >  #define H_PAGE_THP_HUGE  H_PAGE_4K_PFN
> >  
> > -/*
> > - * Used to track subpage group valid if H_PAGE_COMBO is set
> > - * This overloads H_PAGE_F_GIX and H_PAGE_F_SECOND
> > - */
> > -#define H_PAGE_COMBO_VALID	(H_PAGE_F_GIX | H_PAGE_F_SECOND)
> > -
> >  /* PTE flags to conserve for HPTE identification */
> >  #define _PAGE_HPTEFLAGS (H_PAGE_BUSY | H_PAGE_F_SECOND | \
> >  			 H_PAGE_F_GIX | H_PAGE_HASHPTE | H_PAGE_COMBO)
> > diff --git a/arch/powerpc/include/asm/book3s/64/hash.h b/arch/powerpc/include/asm/book3s/64/hash.h
> > index 4e957b0..ac049de 100644
> > --- a/arch/powerpc/include/asm/book3s/64/hash.h
> > +++ b/arch/powerpc/include/asm/book3s/64/hash.h
> > @@ -8,11 +8,8 @@
> >   *
> >   */
> >  #define H_PTE_NONE_MASK		_PAGE_HPTEFLAGS
> > -#define H_PAGE_F_GIX_SHIFT	56
> > -#define H_PAGE_BUSY		_RPAGE_RSV1 /* software: PTE & hash are busy */
> > -#define H_PAGE_F_SECOND		_RPAGE_RSV2	/* HPTE is in 2ndary HPTEG */
> > -#define H_PAGE_F_GIX		(_RPAGE_RSV3 | _RPAGE_RSV4 | _RPAGE_RPN44)
> > -#define H_PAGE_HASHPTE		_RPAGE_RPN43	/* PTE has associated HPTE */
> > +
> > +#define INIT_HIDX (~0x0UL)
> >  
> >  #ifdef CONFIG_PPC_64K_PAGES
> >  #include <asm/book3s/64/hash-64k.h>
> > @@ -160,6 +157,11 @@ static inline int hash__pte_none(pte_t pte)
> >  	return (pte_val(pte) & ~H_PTE_NONE_MASK) == 0;
> >  }
> >  
> > +static inline bool hpte_soft_invalid(unsigned long slot)
> > +{
> > +	return ((slot & 0xfUL) == 0xfUL);
> > +}
> > +
> >  /* This low level function performs the actual PTE insertion
> >   * Setting the PTE depends on the MMU type and other factors. It's
> >   * an horrible mess that I'm not going to try to clean up now but
> > diff --git a/arch/powerpc/mm/hash64_64k.c b/arch/powerpc/mm/hash64_64k.c
> > index 1a68cb1..a16cd28 100644
> > --- a/arch/powerpc/mm/hash64_64k.c
> > +++ b/arch/powerpc/mm/hash64_64k.c
> > @@ -20,29 +20,7 @@
> >   */
> >  bool __rpte_sub_valid(real_pte_t rpte, unsigned long index)
> >  {
> > -	unsigned long g_idx;
> > -	unsigned long ptev = pte_val(rpte.pte);
> > -
> > -	g_idx = (ptev & H_PAGE_COMBO_VALID) >> H_PAGE_F_GIX_SHIFT;
> > -	index = index >> 2;
> > -	if (g_idx & (0x1 << index))
> > -		return true;
> > -	else
> > -		return false;
> > -}
> > -/*
> > - * index from 0 - 15
> > - */
> > -static unsigned long mark_subptegroup_valid(unsigned long ptev, unsigned long index)
> > -{
> > -	unsigned long g_idx;
> > -
> > -	if (!(ptev & H_PAGE_COMBO))
> > -		return ptev;
> > -	index = index >> 2;
> > -	g_idx = 0x1 << index;
> > -
> > -	return ptev | (g_idx << H_PAGE_F_GIX_SHIFT);
> > +	return !(hpte_soft_invalid(rpte.hidx >> (index << 2)));
> >  }
> >  
> >  int __hash_page_4K(unsigned long ea, unsigned long access, unsigned long vsid,
> > @@ -50,12 +28,11 @@ int __hash_page_4K(unsigned long ea, unsigned long access, unsigned long vsid,
> >  		   int ssize, int subpg_prot)
> >  {
> >  	real_pte_t rpte;
> > -	unsigned long *hidxp;
> >  	unsigned long hpte_group;
> >  	unsigned int subpg_index;
> > -	unsigned long rflags, pa, hidx;
> > +	unsigned long rflags, pa;
> >  	unsigned long old_pte, new_pte, subpg_pte;
> > -	unsigned long vpn, hash, slot;
> > +	unsigned long vpn, hash, slot, gslot;
> >  	unsigned long shift = mmu_psize_defs[MMU_PAGE_4K].shift;
> >  
> >  	/*
> > @@ -116,8 +93,8 @@ int __hash_page_4K(unsigned long ea, unsigned long access, unsigned long vsid,
> >  		 * On hash insert failure we use old pte value and we don't
> >  		 * want slot information there if we have a insert failure.
> >  		 */
> > -		old_pte &= ~(H_PAGE_HASHPTE | H_PAGE_F_GIX | H_PAGE_F_SECOND);
> > -		new_pte &= ~(H_PAGE_HASHPTE | H_PAGE_F_GIX | H_PAGE_F_SECOND);
> > +		old_pte &= ~(H_PAGE_HASHPTE);
> > +		new_pte &= ~(H_PAGE_HASHPTE);
> >  		goto htab_insert_hpte;
> >  	}
> >  	/*
> > @@ -148,6 +125,15 @@ int __hash_page_4K(unsigned long ea, unsigned long access, unsigned long vsid,
> >  	}
> >  
> >  htab_insert_hpte:
> > +
> > +	/*
> > +	 * initialize all hidx entries to a invalid value,
> > +	 * the first time the PTE is about to allocate
> > +	 * a 4K hpte
> > +	 */
> > +	if (!(old_pte & H_PAGE_COMBO))
> > +		rpte.hidx = INIT_HIDX;
> > +
> >  	/*
> >  	 * handle H_PAGE_4K_PFN case
> >  	 */
> > @@ -172,15 +158,39 @@ int __hash_page_4K(unsigned long ea, unsigned long access, unsigned long vsid,
> >  	 * Primary is full, try the secondary
> >  	 */
> >  	if (unlikely(slot == -1)) {
> > +		bool soft_invalid;
> > +
> >  		hpte_group = ((~hash & htab_hash_mask) * HPTES_PER_GROUP) & ~0x7UL;
> >  		slot = mmu_hash_ops.hpte_insert(hpte_group, vpn, pa,
> >  						rflags, HPTE_V_SECONDARY,
> >  						MMU_PAGE_4K, MMU_PAGE_4K,
> >  						ssize);
> > -		if (slot == -1) {
> > -			if (mftb() & 0x1)
> > +
> > +		soft_invalid = hpte_soft_invalid(slot);
> > +		if (unlikely(soft_invalid)) {
> > +			/* we got a valid slot from a hardware point of view.
> > +			 * but we cannot use it, because we use this special
> > +			 * value; as defined by hpte_soft_invalid(),
> > +			 * to track invalid slots. We cannot use it.
> > +			 * So invalidate it.
> > +			 */
> 
> Comment style -- needs fixing
> 

hmm.. checkpatch.pl did not catch it.

> > +			gslot = slot & _PTEIDX_GROUP_IX;
> > +			mmu_hash_ops.hpte_invalidate(hpte_group+gslot, vpn,
> > +				MMU_PAGE_4K, MMU_PAGE_4K,
> > +				ssize, 0);
> > +		}
> > +
> > +		if (unlikely(slot == -1 || soft_invalid)) {
> > +			/* for soft invalid slot lets ensure that we
> > +			 * release a slot from the primary, with the
> > +			 * hope that we will acquire that slot next
> > +			 * time we try. This will ensure that we dont
> > +			 * get the same soft-invalid slot.
> > +			 */
> > +			if (soft_invalid || (mftb() & 0x1))
> >  				hpte_group = ((hash & htab_hash_mask) *
> >  					      HPTES_PER_GROUP) & ~0x7UL;
> > +
> >  			mmu_hash_ops.hpte_remove(hpte_group);
> >  			/*
> >  			 * FIXME!! Should be try the group from which we removed ?
> > diff --git a/arch/powerpc/mm/hash_utils_64.c b/arch/powerpc/mm/hash_utils_64.c
> > index f2095ce..1b494d0 100644
> > --- a/arch/powerpc/mm/hash_utils_64.c
> > +++ b/arch/powerpc/mm/hash_utils_64.c
> > @@ -975,8 +975,9 @@ void __init hash__early_init_devtree(void)
> >  
> >  void __init hash__early_init_mmu(void)
> >  {
> > +#ifndef CONFIG_PPC_64K_PAGES
> >  	/*
> > -	 * We have code in __hash_page_64K() and elsewhere, which assumes it can
> > +	 * We have code in __hash_page_4K() and elsewhere, which assumes it can
> >  	 * do the following:
> >  	 *   new_pte |= (slot << H_PAGE_F_GIX_SHIFT) & (H_PAGE_F_SECOND | H_PAGE_F_GIX);
> >  	 *
> > @@ -987,6 +988,7 @@ void __init hash__early_init_mmu(void)
> >  	 * with a BUILD_BUG_ON().
> >  	 */
> >  	BUILD_BUG_ON(H_PAGE_F_SECOND != (1ul  << (H_PAGE_F_GIX_SHIFT + 3)));
> > +#endif /* CONFIG_PPC_64K_PAGES */
> >  
> >  	htab_init_page_sizes();
> >
> 

Thanks for your comments,
RP