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Date: Tue, 11 Jul 2017 15:59:59 +1000
From: Balbir Singh <bsingharora@gmail.com>
Subject: Re: [RFC v5 02/38] powerpc: Free up four 64K PTE bits in 64K backed
 HPTE pages
Message-ID: <20170711155959.79e2d4de@firefly.ozlabs.ibm.com>
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References: <1499289735-14220-1-git-send-email-linuxram@us.ibm.com>
	<1499289735-14220-3-git-send-email-linuxram@us.ibm.com>
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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

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?

Balbir Singh. 

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