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Date:   Fri, 20 Nov 2020 12:18:22 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Peter Zijlstra <peterz@...radead.org>, kan.liang@...ux.intel.com,
        mingo@...nel.org, acme@...nel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        eranian@...gle.com
Cc:     npiggin@...il.com, linuxppc-dev@...ts.ozlabs.org,
        mpe@...erman.id.au, will@...nel.org, willy@...radead.org,
        aneesh.kumar@...ux.ibm.com, sparclinux@...r.kernel.org,
        davem@...emloft.net, catalin.marinas@....com,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        ak@...ux.intel.com, dave.hansen@...el.com,
        kirill.shutemov@...ux.intel.com
Subject: Re: [PATCH 0/5] perf/mm: Fix PERF_SAMPLE_*_PAGE_SIZE

Hi Peter,

Le 13/11/2020 à 14:44, Christophe Leroy a écrit :
> Hi
> 
> Le 13/11/2020 à 12:19, Peter Zijlstra a écrit :
>> Hi,
>>
>> These patches provide generic infrastructure to determine TLB page size from
>> page table entries alone. Perf will use this (for either data or code address)
>> to aid in profiling TLB issues.
>>
>> While most architectures only have page table aligned large pages, some
>> (notably ARM64, Sparc64 and Power) provide non page table aligned large pages
>> and need to provide their own implementation of these functions.
>>
>> I've provided (completely untested) implementations for ARM64 and Sparc64, but
>> failed to penetrate the _many_ Power MMUs. I'm hoping Nick or Aneesh can help
>> me out there.
>>
> 
> I can help with powerpc 8xx. It is a 32 bits powerpc. The PGD has 1024 entries, that means each 
> entry maps 4M.
> 
> Page sizes are 4k, 16k, 512k and 8M.
> 
> For the 8M pages we use hugepd with a single entry. The two related PGD entries point to the same 
> hugepd.
> 
> For the other sizes, they are in standard page tables. 16k pages appear 4 times in the page table. 
> 512k entries appear 128 times in the page table.
> 
> When the PGD entry has _PMD_PAGE_8M bits, the PMD entry points to a hugepd with holds the single 8M 
> entry.
> 
> In the PTE, we have two bits: _PAGE_SPS and _PAGE_HUGE
> 
> _PAGE_HUGE means it is a 512k page
> _PAGE_SPS means it is not a 4k page
> 
> The kernel can by build either with 4k pages as standard page size, or 16k pages. It doesn't change 
> the page table layout though.
> 
> Hope this is clear. Now I don't really know to wire that up to your series.

Does my description make sense ? Is there anything I can help with ?

Christophe

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