Re: [PATCH 2/2] perf/x86/intel/pt: Fail event creation if VMX operation is on

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Tue, Feb 14, 2017 at 03:24:16PM +0200, Alexander Shishkin escreveu:
> On systems where PT does not coexist with VMX, users get confused when
> PT turns up with no data because they forgot they're running a kvm
> session at the same time.
> 
> This patch adds a preemptive check for any active VMX operations that
> will fail event creation. This does not provide any guarantees or
> protection against racing with a kvm starting in parallel, but is
> intended to serve as a hint for the user. If VMXON happens after an
> event had been created, the event will still produce an empty trace.
> 
> Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Reported-by: Andi Kleen <ak@linux.intel.com>

Yeah, I saw that as well, and Andi told me about this limitation, so,
for quite a while now, everytime I need to test PT on the only machine I
have with it, I have to stop my kvm sessions :-\

Thanks for working on this!

- Arnaldo

> ---
>  arch/x86/events/intel/pt.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/arch/x86/events/intel/pt.c b/arch/x86/events/intel/pt.c
> index 9372fa4549..b1490a879c 100644
> --- a/arch/x86/events/intel/pt.c
> +++ b/arch/x86/events/intel/pt.c
> @@ -1444,6 +1444,20 @@ static void pt_event_destroy(struct perf_event *event)
>  
>  static int pt_event_init(struct perf_event *event)
>  {
> +	int cpu, vmx_on = 0;
> +
> +	get_online_cpus();
> +	for_each_online_cpu(cpu) {
> +		struct pt *pt = per_cpu_ptr(&pt_ctx, cpu);
> +
> +		if (READ_ONCE(pt->vmx_on))
> +			vmx_on++;
> +	}
> +	put_online_cpus();
> +
> +	if (vmx_on)
> +		return -EBUSY;
> +
>  	if (event->attr.type != pt_pmu.pmu.type)
>  		return -ENOENT;
>  
> -- 
> 2.11.0