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

[Thomas Gleixner]

On Thu, 23 Mar 2017, Peter Zijlstra wrote:
> On Thu, Mar 23, 2017 at 11:25:49AM -0700, kan.liang@xxxxxxxxx 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