linux-kernel - Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access fails without !panic_on

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Message-ID: <20150930140122.GB3285@gmail.com>
Date:	Wed, 30 Sep 2015 16:01:22 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paolo Bonzini <pbonzini@...hat.com>,
	xen-devel <Xen-devel@...ts.xen.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KVM list <kvm@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access
 fails without !panic_on_oops


* Peter Zijlstra <peterz@...radead.org> wrote:

> 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@...nel.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.

These could still be open coded in an inlined fashion, like the scheduler usage.

Thanks,

	Ingo
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