From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe"
 MSR access fails without !panic_on_oops
Date: Wed, 30 Sep 2015 15:10:02 +0200
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: 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>, xen-devel <Xen-devel@lists.xen.org>, Paolo Bonzini <pbonzini@redhat.com>, Thomas Gleixner <tglx@linutronix.de>, Andy Lutomirski <luto@amacapital.net>, Arjan van de Ven <arjan@linux.intel.com>, Ingo Molnar <mingo@kernel.org>
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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.