Date: Sun, 18 May 2008 14:06:50 -0400 From: Mike Meyer <mwm-keyword-freebsdhackers2.e313df@mired.org> To: Rui Paulo <rpaulo@FreeBSD.org> Cc: freebsd-hackers@freebsd.org, Mike Meyer <mwm-keyword-freebsdhackers2.e313df@mired.org> Subject: Re: rdmsr from userspace Message-ID: <20080518140650.2691af1c@bhuda.mired.org> In-Reply-To: <48305044.7000007@FreeBSD.org> References: <482E93C0.4070802@icyb.net.ua> <20080517133037.3a3935db@bhuda.mired.org> <48305044.7000007@FreeBSD.org>
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On Sun, 18 May 2008 16:50:28 +0100
Rui Paulo <rpaulo@FreeBSD.org> wrote:
> Mike Meyer wrote:
> > On Sat, 17 May 2008 11:13:52 +0300
> > Andriy Gapon <avg@icyb.net.ua> wrote:
> >> It seems that rdmsr instruction can be executed only at the highest
> >> privilege level and thus is not permitted from userland. Maybe we should
> >> provide something like Linux /dev/cpu/msr?
> >> I don't like interface of that device, I think that ioctl approach would
> >> be preferable in this case.
> >> Something like create /dev/cpuN and allow some ioctls on it:
> >> ioctl(cpu_fd, CPU_RDMSR, arg).
> >> What do you think?
> >
> > Ok, this points directly at a question I've been wondering about, but
> > haven't been able to find an answer in the google.
> >
> > I've been mucking about with general access to sysctl's (a sysctl
> > plugin for gkrellm, and a python module for accessing sysctls), and
> > with that hammer in my hand, the nail for this problem is obviously a
> > dev.cpu.#.msr sysctl.
>
> How can you request a rdmsr within the sysctl tree? I don't think sysctl
> is appropriate here either.
Reading (or writing) a sysctl mib can trigger a sysctl handler, which
can do pretty much anything. In particular, there are already examples
in the kernel where sysctl handlers use devices that don't have /dev
entries to get & set their values. Look through kern/kern_cpu.c and
i386/cpufreq/p4tcc.c to see the two ends of that kind of
connection. In fact, the cpu frequency sysctls would seem to be an
excellent model for something like the msr.
ioctl, open+read/write, sysctl - they're all just interfaces to kernel
handlers.
<mike
--
Mike Meyer <mwm@mired.org> http://www.mired.org/consulting.html
Independent Network/Unix/Perforce consultant, email for more information.
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