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

From: Balbir Singh <bsingharora@gmail.com>
To: Ram Pai <linuxram@us.ibm.com>
Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	linux-arch@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
	linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org,
	benh@kernel.crashing.org, paulus@samba.org, mpe@ellerman.id.au,
	khandual@linux.vnet.ibm.com, aneesh.kumar@linux.vnet.ibm.com,
	dave.hansen@intel.com, hbabu@us.ibm.com, arnd@arndb.de,
	akpm@linux-foundation.org, corbet@lwn.net, mingo@redhat.com
Subject: Re: [RFC v5 02/38] powerpc: Free up four 64K PTE bits in 64K backed HPTE pages
Date: Wed, 12 Jul 2017 13:10:51 +1000	[thread overview]
Message-ID: <20170712131051.5bf657c7@firefly.ozlabs.ibm.com> (raw)
In-Reply-To: <20170711154415.GA5525@ram.oc3035372033.ibm.com>

On Tue, 11 Jul 2017 08:44:15 -0700
Ram Pai <linuxram@us.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@us.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.

Balbir Singh.

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