linux-kernel - Re: [RFC v2 02/12] powerpc: Free up four 64K PTE bits in 64K backed hpte pages.

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Date:   Tue, 20 Jun 2017 16:21:45 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Ram Pai <linuxram@...ibm.com>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Cc:     dave.hansen@...el.com, paulus@...ba.org,
        aneesh.kumar@...ux.vnet.ibm.com
Subject: Re: [RFC v2 02/12] powerpc: Free up four 64K PTE bits in 64K backed
 hpte pages.

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 64K backed hpte pages. This along with the earlier
> patch will entirely free up the four bits from 64K PTE.
> 
> This patch does the following change to 64K PTE that is
> backed by 64K hpte.
> 
> 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.
> 
> since bit 7 is now freed up, we move H_PAGE_BUSY from bit 9
> to bit 7. Trying to minimize gaps so that contiguous bits
> can be allocated if needed in the future.
> 
> The second part of the PTE will hold
> (H_PAGE_F_SECOND|H_PAGE_F_GIX) at bit 60,61,62,63.

I still dont understand how we freed up the 5th bit which is
used in the 5th patch. Was that bit never used for any thing
on 64K page size (64K and 4K mappings) ?

+#define _RPAGE_RSV5		0x00040UL

+#define H_PAGE_PKEY_BIT0	_RPAGE_RSV1
+#define H_PAGE_PKEY_BIT1	_RPAGE_RSV2
+#define H_PAGE_PKEY_BIT2	_RPAGE_RSV3
+#define H_PAGE_PKEY_BIT3	_RPAGE_RSV4
+#define H_PAGE_PKEY_BIT4	_RPAGE_RSV5