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Subject: Re: [PATCH V9 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE
To:     Will Deacon <will@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     mingo@redhat.com, acme@kernel.org, linux-kernel@vger.kernel.org,
        mark.rutland@arm.com, alexander.shishkin@linux.intel.com,
        jolsa@redhat.com, eranian@google.com, ak@linux.intel.com,
        dave.hansen@intel.com, kirill.shutemov@linux.intel.com,
        benh@kernel.crashing.org, paulus@samba.org,
        David Miller <davem@davemloft.net>
References: <20201001135749.2804-1-kan.liang@linux.intel.com>
 <20201001135749.2804-2-kan.liang@linux.intel.com>
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 <877drz1qbc.fsf@mpe.ellerman.id.au> <20201012084829.GA1151@willie-the-truck>
From:   "Liang, Kan" <kan.liang@linux.intel.com>
Message-ID: <de47984b-9a69-733c-3bd1-7b24ceb9b7f0@linux.intel.com>
Date:   Tue, 13 Oct 2020 10:57:41 -0400
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On 10/12/2020 4:48 AM, Will Deacon wrote:
> On Sat, Oct 10, 2020 at 12:28:39AM +1100, Michael Ellerman wrote:
>> Peter Zijlstra <peterz@infradead.org> writes:
>>> Patch 4 makes it all far worse by exposing it to pretty much everybody.
>>>
>>> Now, I think we can fix at least the user mappings with the below delta,
>>> but if archs are using non-page-table MMU sizes we'll need arch helpers.
>>>
>>> ARM64 is in that last boat.
>>
>> I think we probably need it to be weak so we can provide our own
>> version.
> 
> I guess the same thing applies to us, but I can't really tell how accurate
> this stuff needs to be for userspace. If it's trying to use the page-table
> configuration to infer properties of the TLB, that's never going to be
> reliable because the TLB can choose both to split and coalesce entries
> as long as software can't tell.
> 

Hi Peter,

It looks like everybody wants a __weak function. If so, I guess we 
should drop the generic code in this patch. For X86, we have existing 
functions to retrieve the page level and the page size. I think we don't 
need the generic code either.
https://lkml.kernel.org/r/1549648509-12704-2-git-send-email-kan.liang@linux.intel.com/

Should I send a V10 patch to drop the generic code and implement an X86 
specific perf_get_page_size()? Will, Michael, and others can implement 
their version later separately.

Thanks,
Kan