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From: Andy Lutomirski <luto@amacapital.net>
Date: Wed, 1 Jul 2015 10:33:54 -0700
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Subject: Re: [PATCH] x86, msr: Allow read access to /dev/cpu/X/msr
To: "Brown, Len" <len.brown@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>, Prarit Bhargava <prarit@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, X86 ML <x86@kernel.org>,
        "Chandramouli, Dasaratharaman" 
	<dasaratharaman.chandramouli@intel.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
        Denys Vlasenko <dvlasenk@redhat.com>, Brian Gerst <brgerst@gmail.com>,
        Arnaldo Carvalho de Melo <acme@infradead.org>
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On Wed, Jul 1, 2015 at 9:38 AM, Brown, Len <len.brown@intel.com> wrote:
> BTW. I've had a discussion w/ LLNL about their needs,
> both for security and performance.  For security, as concluded
> by this thread, a white list is the only way to go.
> I'm thinking a bit-vector of allowed MSR offsets...
> For performance, they absolutely can not afford a system call
> for every single MSR access.

I'm surprised.  On a sane kernel, a syscall is about 120 cycles.  Just
rdmsr to an unoptimized MSR is probably fifty cycles, I'd guess.

Of course, LLNL is probably using NOHZ_FULL, which is currently very,
very slow.  Work is afoot to fix that.

>  Here an ioctl to have the
> msr driver perform a vector of accesses in a single system
> call seems the way to go.  I can prototype both of these
> using turbostat as the customer.

How about preadv?

--Andy
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