Re: [PATCH 3/6] perf/x86/intel/pt: Stop tracing at syscore shutdown

[Adrian Hunter]

On 02/11/2026 12:04, Peter Zijlstra wrote:
> On Mon, Nov 02, 2026 at 09:40:36AM +0200, Adrian Hunter wrote:
>> +static void pt_shutdown_cpu(void *info)
>> +{
>> +	u64 ctl;
>> +
>> +	/*
>> +	 * After VMXON on CPUs without PT support in VMX operation,
>> +	 * TraceEn is already clear and RTIT_CTL must not be written.
>> +	 */
>> +	rdmsrq(MSR_IA32_RTIT_CTL, ctl);
>> +	if (ctl & RTIT_CTL_TRACEEN)
>> +		wrmsrq(MSR_IA32_RTIT_CTL, ctl & ~RTIT_CTL_TRACEEN);
>> +}
> 
> Why not unconditionally do: wrmsrq(MSR_IA32_RTIT_CTL, 0), at this point?

Writing 0 instead of ctl & ~RTIT_CTL_TRACEEN would be fine, but the
condition is needed.  The comment tries to explain.

The first processor to support Intel PT (BDW) will #GP if a write is
made to RTIT_CTL while VMX is on.  Since VMXON clears TRACEEN,
if TRACEEN is set, then the write is safe.