linux-kernel - Re: [PATCH v4 00/10] riscv: Allow userspace to directly access perf counters

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Date:   Tue, 18 Jul 2023 22:04:45 +0300
From:   Rémi Denis-Courmont <remi@...lab.net>
To:     Atish Patra <atishp@...shpatra.org>
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Aurelien Jarno <aurelien@...el32.net>,
        Mathieu Malaterre <malat@...ian.org>,
        Jan Newger <jannewger@...gle.com>
Subject: Re: [PATCH v4 00/10] riscv: Allow userspace to directly access perf counters

Le tiistaina 18. heinäkuuta 2023, 21.45.15 EEST Atish Patra a écrit :
> > I agree that it's not only insecure but also incorrect. However it mostly
> > works. In fact I don't disagree with the change as such, but I think that
> > the commit messages are misleading and confusing. For a start, in one
> > place it says that it is not breaking user space and in another it says
> > basically the opposite.
> 
> Agreed. We will improve the commit message to clarify that. That's also the
> reason I started this whole thread :)
> 
> > (Unfortunately, not everybody agrees with the change. I can't seem to get
> > FFmpeg's checkasm tool fixed:
> > http://ffmpeg.org/pipermail/ffmpeg-devel/2023-July/312245.html )
> 
> Why can't rdtime(equivalent of rdtsc) be used instead of rdcycle ?

Isn't RDTIM susceptible to interference from power management and CPU 
frequency scaling? I suppose that RDCYCLE may behave differently depending on 
PM in *some* designs, but that would still be way better than RDTIME for the 
purpose.

As far as benchmarking is concerned (_excluding_ system security), RDTIME 
seems to have all the problems of RDCYCLE, and then some more, no?

> The perf syscall overhead is just one time setup thing during the
> start of the application.
> For counting the cycles before/after a loop, it still provides a
> direct CSR access in user mode.

I suppose that you allude to mmap() here? The (dumb) FFmpeg code is using the 
ioctl() interface though, but that's just laziness.

-- 
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