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 09:31:25 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     kan.liang@...el.com, mingo@...hat.com, acme@...nel.org,
        linux-kernel@...r.kernel.org, eranian@...gle.com, jolsa@...nel.org,
        ak@...ux.intel.com
Subject: Re: [PATCH 1/3] perf/x86: add sysfs entry to freeze counter on SMI

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.

Thanks,

	tglx