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

From: Alexander Shishkin
Date: Wed Feb 15 2017 - 03:37:43 EST

Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> writes:

> 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@xxxxxxxxxxxxxxx>
>> Reported-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> 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!

Well, we can't make the limitation go away. You'll still have to stop
kvms to get any 'meaningful' PT data.