linux-kernel - Re: [RFC v5 02/38] powerpc: Free up four 64K PTE bits in 64K backed HPTE pages

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Message-Id: <20170713073941.GE5525@ram.oc3035372033.ibm.com>
Date:   Thu, 13 Jul 2017 00:39:41 -0700
From:   Ram Pai <linuxram@...ibm.com>
To:     Balbir Singh <bsingharora@...il.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
        benh@...nel.crashing.org, paulus@...ba.org, mpe@...erman.id.au,
        khandual@...ux.vnet.ibm.com, aneesh.kumar@...ux.vnet.ibm.com,
        dave.hansen@...el.com, hbabu@...ibm.com, arnd@...db.de,
        akpm@...ux-foundation.org, corbet@....net, mingo@...hat.com
Subject: Re: [RFC v5 02/38] powerpc: Free up four 64K PTE bits in 64K backed
 HPTE pages

On Wed, Jul 12, 2017 at 01:10:51PM +1000, Balbir Singh wrote:
> On Tue, 11 Jul 2017 08:44:15 -0700
> Ram Pai <linuxram@...ibm.com> wrote:
> 
> > On Tue, Jul 11, 2017 at 03:59:59PM +1000, Balbir Singh wrote:
> > > On Wed,  5 Jul 2017 14:21:39 -0700
> > > Ram Pai <linuxram@...ibm.com> wrote:
> > >   
> > > > 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.

Yes. the later.
RP