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From:   Vince Weaver <vincent.weaver@maine.edu>
Date:   Sun, 29 Aug 2021 23:05:55 -0400 (EDT)
To:     Andy Lutomirski <luto@kernel.org>
cc:     Rob Herring <robh@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Will Deacon <will@kernel.org>,
        Kan Liang <kan.liang@linux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        the arch/x86 maintainers <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        linux-perf-users@vger.kernel.org
Subject: Re: [RFC 2/3] perf/x86: Control RDPMC access from .enable() hook
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References: <20210728230230.1911468-1-robh@kernel.org> <20210728230230.1911468-3-robh@kernel.org> <d720903c-926e-f57a-0862-4e5d76db763a@kernel.org> <CAL_JsqLRv9ugKJcn4dq_ps0JMt74Y7PKA=5yySYxvftdQWzzPA@mail.gmail.com> <de97454b-9b4d-492f-a435-6a5e33889219@www.fastmail.com>
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On Fri, 27 Aug 2021, Andy Lutomirski wrote:

> On Thu, Aug 26, 2021, at 12:09 PM, Rob Herring wrote:

> > After testing some scenarios and finding perf_event_tests[1], this
> > series isn't going to work for x86 unless rdpmc is restricted to task
> > events only or allowed to segfault on CPU events when read on the
> > wrong CPU rather than just returning garbage. It's been discussed
> > before here[2].
> > 
> > Ultimately, I'm just trying to define the behavior for arm64 where we
> > don't have an existing ABI to maintain and don't have to recreate the
> > mistakes of x86 rdpmc ABI. Tying the access to mmap is messy. As we
> > explicitly request user access on perf_event_open(), I think it may be
> > better to just enable access when the event's context is active and
> > ignore mmap(). Maybe you have an opinion there since you added the
> > mmap() part?
> 
> That makes sense to me. The mmap() part was always a giant kludge.
> 
> There is fundamentally a race, at least if rseq isn’t used: if you check 
> that you’re on the right CPU, do RDPMC, and throw out the result if you 
> were on the wrong CPU (determined by looking at the mmap), you still 
> would very much prefer not to fault.
> 
> Maybe rseq or a vDSO helper is the right solution for ARM.

as the author of those perf_event tests for rdpmc, I have to say if ARM 
comes up with a cleaner implementation I'd be glad to have x86 transition 
to something better.

The rdpmc code is a huge mess and has all kinds of corner cases.  I'm not 
sure anyone besides the PAPI library tries to use it, and while it's a 
nice performance improvement to use rdpmc it is really hard to get things 
working right.

As a PAPI developer we actually have run into the issue where the CPU 
switches and we were reporting the wrong results.  Also if I recall (it's 
been a while) we were having issues where the setup lets you attach to a 
process on another CPU for monitoring using the rdpmc interface and it 
returns results even though I think that will rarely ever work in 
practice.

Vince
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