linux-kernel - Re: [PATCH 1/3] perf/x86: add sysfs entry to freeze counter on SMI

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Date:   Fri, 24 Mar 2017 15:23:29 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liang, Kan" <kan.liang@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "acme@...nel.org" <acme@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "eranian@...gle.com" <eranian@...gle.com>,
        "jolsa@...nel.org" <jolsa@...nel.org>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: Re: [PATCH 1/3] perf/x86: add sysfs entry to freeze counter on SMI

On Fri, Mar 24, 2017 at 02:15:16PM +0000, Liang, Kan wrote:
> 
> 
> > On Thu, 23 Mar 2017, Peter Zijlstra wrote:
> > > On Thu, Mar 23, 2017 at 11:25:49AM -0700, kan.liang@...el.com wrote:
> > > > +	for_each_possible_cpu(cpu) {
> > > > +		rdmsrl_on_cpu(cpu, MSR_IA32_DEBUGCTLMSR,
> > &debugctlmsr);
> > > > +		if (val)
> > > > +			wrmsrl_on_cpu(cpu, MSR_IA32_DEBUGCTLMSR,
> > debugctlmsr | DEBUGCTLMSR_FREEZE_WHILE_SMM);
> > > > +		else
> > > > +			wrmsrl_on_cpu(cpu, MSR_IA32_DEBUGCTLMSR,
> > debugctlmsr & ~DEBUGCTLMSR_FREEZE_WHILE_SMM);
> > > > +	}
> > >
> > > No; that's just disgusting. Also {rd,wr}msr_on_cpu() should die,
> > > exactly because people end up writing crap like this.
> > 
> > Aside of that this is completely broken against other users of
> > DEBUGCTLMSR because it's not atomic vs. the other modifications.
> > 
> 
> OK. I will change it.
> I guess I need a way/function which can atomically rd,wr msr on all cpus.
> Are there existing alternative ways/functions to do that?

No; also don't forget hotplug.