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From: Andy Lutomirski <luto@amacapital.net>
Date: Thu, 2 Jul 2015 12:26:04 -0700
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Subject: Re: [PATCH] x86, msr: Allow read access to /dev/cpu/X/msr
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: "Brown, Len" <len.brown@intel.com>, Ingo Molnar <mingo@kernel.org>,
        Prarit Bhargava <prarit@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        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 Thu, Jul 2, 2015 at 12:22 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 07/01/2015 09:38 AM, Brown, Len 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.  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.
>>
>
> Every time I have heard about people having issues with performance for
> MSR access, it is because they are doing cross-CPU accesses which means
> a neverending stream of IPIs.  You get immensely better performance by
> tying a thread to a CPU and only accessing the local CPU from that
> thread.  This has addressed any performance problems anyone has ever
> come to me with.  As Andy and Ingo have already pointed out, the MSR
> access itself is pretty much as expensive as the system call overhead.

To be fair, before we had opportunistic sysret,
CONFIG_CONTEXT_TRACKING was *extremely* expensive.  Even now, it's
still pretty bad.

Len, do you know what configuration and kernel version this was on or
what the apparent syscall overhead was?

--Andy
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