linux-kernel - Re: [PATCH V9 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE

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Message-ID: <20201012084829.GA1151@willie-the-truck>
Date:   Mon, 12 Oct 2020 09:48:29 +0100
From:   Will Deacon <will@...nel.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Peter Zijlstra <peterz@...radead.org>, kan.liang@...ux.intel.com,
        mingo@...hat.com, acme@...nel.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, eranian@...gle.com, ak@...ux.intel.com,
        dave.hansen@...el.com, kirill.shutemov@...ux.intel.com,
        benh@...nel.crashing.org, paulus@...ba.org,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH V9 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE

On Sat, Oct 10, 2020 at 12:28:39AM +1100, Michael Ellerman wrote:
> Peter Zijlstra <peterz@...radead.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.

Will