Re: [PATCH V2 02/11] perf/x86: Add support for TSC as a perf event clock

[Adrian Hunter]

On 04/03/2022 14:30, Peter Zijlstra wrote:
> On Mon, Feb 14, 2022 at 01:09:05PM +0200, Adrian Hunter wrote:
>> Currently, using Intel PT to trace a VM guest is limited to kernel space
>> because decoding requires side band events such as MMAP and CONTEXT_SWITCH.
>> While these events can be collected for the host, there is not a way to do
>> that yet for a guest. One approach, would be to collect them inside the
>> guest, but that would require being able to synchronize with host
>> timestamps.
>>
>> The motivation for this patch is to provide a clock that can be used within
>> a VM guest, and that correlates to a VM host clock. In the case of TSC, if
>> the hypervisor leaves rdtsc alone, the TSC value will be subject only to
>> the VMCS TSC Offset and Scaling. Adjusting for that would make it possible
>> to inject events from a guest perf.data file, into a host perf.data file.
>>
>> Thus making possible the collection of VM guest side band for Intel PT
>> decoding.
>>
>> There are other potential benefits of TSC as a perf event clock:
>> 	- ability to work directly with TSC
>> 	- ability to inject non-Intel-PT-related events from a guest
>>
>> Signed-off-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
>> ---
>>  arch/x86/events/core.c            | 16 +++++++++
>>  arch/x86/include/asm/perf_event.h |  3 ++
>>  include/uapi/linux/perf_event.h   | 12 ++++++-
>>  kernel/events/core.c              | 57 +++++++++++++++++++------------
>>  4 files changed, 65 insertions(+), 23 deletions(-)
>>
>> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
>> index e686c5e0537b..51d5345de30a 100644
>> --- a/arch/x86/events/core.c
>> +++ b/arch/x86/events/core.c
>> @@ -2728,6 +2728,17 @@ void arch_perf_update_userpage(struct perf_event *event,
>>  		!!(event->hw.flags & PERF_EVENT_FLAG_USER_READ_CNT);
>>  	userpg->pmc_width = x86_pmu.cntval_bits;
>>  
>> +	if (event->attr.use_clockid &&
>> +	    event->attr.ns_clockid &&
>> +	    event->attr.clockid == CLOCK_PERF_HW_CLOCK) {
>> +		userpg->cap_user_time_zero = 1;
>> +		userpg->time_mult = 1;
>> +		userpg->time_shift = 0;
>> +		userpg->time_offset = 0;
>> +		userpg->time_zero = 0;
>> +		return;
>> +	}
>> +
>>  	if (!using_native_sched_clock() || !sched_clock_stable())
>>  		return;
> 
> This looks the wrong way around. If TSC is found unstable, we should
> never expose it.

Intel PT traces contain TSC whether or not it is stable, and it could
still be usable in some cases e.g. short traces on a single CPU.

Ftrace seems to offer x86-tsc unconditionally as a clock.

We could add warnings to comments and documentation about its potential
pitfalls.

> 
> And I'm not at all sure about the whole virt thing. Last time I looked
> at pvclock it made no sense at all.

It is certainly not useful for synchronizing events against TSC.