Re: [PATCH v6 06/16] KVM: x86/pmu: Add IA32_PEBS_ENABLE MSR emulation for extended PEBS

["Xu, Like" <like.xu@intel.com>]

On 2021/5/17 16:32, Peter Zijlstra wrote:
> On Tue, May 11, 2021 at 10:42:04AM +0800, Like Xu wrote:
>> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
>> index 2f89fd599842..c791765f4761 100644
>> --- a/arch/x86/events/intel/core.c
>> +++ b/arch/x86/events/intel/core.c
>> @@ -3898,31 +3898,49 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void *data)
>>   	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>>   	struct perf_guest_switch_msr *arr = cpuc->guest_switch_msrs;
>>   	u64 intel_ctrl = hybrid(cpuc->pmu, intel_ctrl);
>> +	u64 pebs_mask = (x86_pmu.flags & PMU_FL_PEBS_ALL) ?
>> +		cpuc->pebs_enabled : (cpuc->pebs_enabled & PEBS_COUNTER_MASK);
>> +
>> +	*nr = 0;
>> +	arr[(*nr)++] = (struct perf_guest_switch_msr){
>> +		.msr = MSR_CORE_PERF_GLOBAL_CTRL,
>> +		.host = intel_ctrl & ~cpuc->intel_ctrl_guest_mask,
>> +		.guest = intel_ctrl & (~cpuc->intel_ctrl_host_mask | ~pebs_mask),
>> +	};
>>   
>> +	if (!x86_pmu.pebs)
>> +		return arr;
>>   
>> +	/*
>> +	 * If PMU counter has PEBS enabled it is not enough to
>> +	 * disable counter on a guest entry since PEBS memory
>> +	 * write can overshoot guest entry and corrupt guest
>> +	 * memory. Disabling PEBS solves the problem.
>> +	 *
>> +	 * Don't do this if the CPU already enforces it.
>> +	 */
>> +	if (x86_pmu.pebs_no_isolation) {
>> +		arr[(*nr)++] = (struct perf_guest_switch_msr){
>> +			.msr = MSR_IA32_PEBS_ENABLE,
>> +			.host = cpuc->pebs_enabled,
>> +			.guest = 0,
>> +		};
>> +		return arr;
>>   	}
>>   
>> +	if (!x86_pmu.pebs_vmx)
>> +		return arr;
>> +
>> +	arr[*nr] = (struct perf_guest_switch_msr){
>> +		.msr = MSR_IA32_PEBS_ENABLE,
>> +		.host = cpuc->pebs_enabled & ~cpuc->intel_ctrl_guest_mask,
>> +		.guest = pebs_mask & ~cpuc->intel_ctrl_host_mask,
>> +	};
>> +
>> +	/* Set hw GLOBAL_CTRL bits for PEBS counter when it runs for guest */
>> +	arr[0].guest |= arr[*nr].guest;
>> +
>> +	++(*nr);
>>   	return arr;
>>   }
> ISTR saying I was confused as heck by this function, I still don't see
> clarifying comments :/
>
> What's .host and .guest ?

Will adding the following comments help you ?

+/*
+ * Currently, the only caller of this function is the 
atomic_switch_perf_msrs().
+ * The host perf conext helps to prepare the values of the real hardware for
+ * a set of msrs that need to be switched atomically in a vmx transaction.
+ *
+ * For example, the pseudocode needed to add a new msr should look like:
+ *
+ * arr[(*nr)++] = (struct perf_guest_switch_msr){
+ *     .msr = the hardware msr address,
+ *     .host = the value the hardware has when it doesn't run a guest,
+ *     .guest = the value the hardware has when it runs a guest,
+ * };
+ *
+ * These values have nothing to do with the emulated values the guest sees
+ * when it uses {RD,WR}MSR, which should be handled in the KVM context.
+ */
  static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void 
*data)