From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S932994AbbI3NKo (ORCPT <rfc822;w@1wt.eu>);
	Wed, 30 Sep 2015 09:10:44 -0400
Received: from bombadil.infradead.org ([198.137.202.9]:59364 "EHLO
	bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1753078AbbI3NKm (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 30 Sep 2015 09:10:42 -0400
Date: Wed, 30 Sep 2015 15:10:02 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>, Andy Lutomirski <luto@amacapital.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paolo Bonzini <pbonzini@redhat.com>,
        xen-devel <Xen-devel@lists.xen.org>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        KVM list <kvm@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access
 fails without !panic_on_oops
Message-ID: <20150930131002.GK2881@worktop.programming.kicks-ass.net>
References: <cover.1442793572.git.luto@kernel.org>
 <130a3b7ef4788baae3a6fe71293ab17442bc9a0a.1442793572.git.luto@kernel.org>
 <CA+55aFy4Chd-PJvxGne=TztPsvFqUG07Ht8mtc4+0oGNZNDOyw@mail.gmail.com>
 <CALCETrXqsKcqzfqMO2ctOJJ4X8Kb6vvoUpVDir9F2-a3XwK23w@mail.gmail.com>
 <20150921084642.GA30984@gmail.com>
 <CA+55aFzKCi6pf0RP8HjDQYDsms6reB5AihuCAHEkVJtoOHk_Yw@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CA+55aFzKCi6pf0RP8HjDQYDsms6reB5AihuCAHEkVJtoOHk_Yw@mail.gmail.com>
User-Agent: Mutt/1.5.22.1 (2013-10-16)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, Sep 21, 2015 at 09:36:15AM -0700, Linus Torvalds wrote:
> On Mon, Sep 21, 2015 at 1:46 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > Linus, what's your preference?
> 
> So quite frankly, is there any reason we don't just implement
> native_read_msr() as just
> 
>    unsigned long long native_read_msr(unsigned int msr)
>    {
>       int err;
>       unsigned long long val;
> 
>       val = native_read_msr_safe(msr, &err);
>       WARN_ON_ONCE(err);
>       return val;
>    }
> 
> Note: no inline, no nothing. Just put it in arch/x86/lib/msr.c, and be
> done with it. I don't see the downside.
> 
> How many msr reads are <i>so</i> critical that the function call
> overhead would matter? Get rid of the inline version of the _safe()
> thing too, and put that thing there too.

There are a few in the perf code, and esp. on cores without a stack
engine the call overhead is noticeable. Also note that the perf MSRs are
generally optimized MSRs and less slow (we cannot say fast, they're
still MSRs) than regular MSRs.