linux-kernel - Re: [PATCH] x86, msr: Allow read access to /dev/cpu/X/msr

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Date:	Thu, 2 Jul 2015 12:26:04 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"Brown, Len" <len.brown@...el.com>, Ingo Molnar <mingo@...nel.org>,
	Prarit Bhargava <prarit@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
	"Chandramouli, Dasaratharaman" 
	<dasaratharaman.chandramouli@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...nel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>
Subject: Re: [PATCH] x86, msr: Allow read access to /dev/cpu/X/msr

On Thu, Jul 2, 2015 at 12:22 PM, H. Peter Anvin <hpa@...or.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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