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

[Peter Zijlstra]

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.

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