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From:   Jann Horn <jannh@google.com>
Date:   Fri, 28 Sep 2018 23:22:37 +0200
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Subject: Re: [RFC 0/5] perf: Per PMU access controls (paranoid setting)
To:     Andi Kleen <ak@linux.intel.com>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>, tursulin@ursulin.net,
        kernel list <linux-kernel@vger.kernel.org>,
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        "the arch/x86 maintainers" <x86@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>, acme@kernel.org,
        alexander.shishkin@linux.intel.com, jolsa@redhat.com,
        namhyung@kernel.org, maddy@linux.vnet.ibm.com,
        alexey.budankov@linux.intel.com, Kees Cook <keescook@chromium.org>
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On Fri, Sep 28, 2018 at 10:59 PM Andi Kleen <ak@linux.intel.com> wrote:
> > > This new file descriptor argument doesn't exist today so it would
> > > need to create a new system call with more arguments
> >
> > Is that true? The first argument is a pointer to a struct that
> > contains its own size, so it can be expanded without an ABI break. I
> > don't see any reason why you couldn't cram more stuff in there.
>
> You're right we could put the fd into the perf_event, but the following is
> still true:
>
> > > Obviously we would need to keep the old system call around
> > > for compability, so you would need to worry about this
> > > interaction in any case!

<blasphemy>
Is that true? IIRC if you want to use the perf tools after a kernel
update, you have to install a new version of perf anyway, no? I think
after I run a kernel update, when I run "perf top", it just refuses to
start and tells me to go install a newer version. Would the users of
perf_event_open() that want to monitor this graphics stuff normally
keep working after a kernel version bump?
I realize that the kernel is very much against breaking userspace
interfaces, but if userspace has already decided to break itself after
every update, we might as well take advantage of that...
</blasphemy>

> > > So tying it together doesn't make any sense, because
> > > the problem has to be solved separately anyways.