Re: [PATCH v3 6/9] kvm: arm/arm64: Add host pmu to support VM introspection

[Punit Agrawal]

Mark Rutland <mark.rutland@xxxxxxx> writes:

> On Wed, Jan 18, 2017 at 11:21:21AM +0000, Marc Zyngier wrote:
>> On 10/01/17 11:38, Punit Agrawal wrote:
>> > +#define VM_MASK	GENMASK_ULL(31, 0)
>> > +#define EVENT_MASK	GENMASK_ULL(32, 39)
>> > +#define EVENT_SHIFT	(32)
>> > +
>> > +#define to_pid(cfg)	((cfg) & VM_MASK)
>> > +#define to_event(cfg)	(((cfg) & EVENT_MASK) >> EVENT_SHIFT)
>> > +
>> > +PMU_FORMAT_ATTR(vm, "config:0-31");
>> > +PMU_FORMAT_ATTR(event, "config:32-39");
>> 
>> I'm a bit confused by these. Can't you get the PID of the VM you're
>> tracing directly from perf, without having to encode things?

With perf attached to a PID, the event gets scheduled out when the task
is context switched. As the PID of the controlling process was used,
none of the vCPU events were counted.

> And if you
>> can't, surely this should be a function of the size of pid_t?

Agreed. I'll update above if we decide to carry on with this
approach. More below...

>>
>> Mark, can you shine some light here?
>
> AFAICT, this is not necessary.
>
> The perf_event_open() syscall takes a PID separately from the
> perf_event_attr. i.e. we should be able to do:
>
> // monitor a particular vCPU
> perf_event_open(attr, vcpupid, -1, -1, 0)
>
> ... or .. 
>
> // monitor a particular vCPU on a pCPU
> perf_event_open(attr, vcpupid, cpu, -1, 0)
>
> ... or ...
>
> // monitor all vCPUs on a pCPU
> perf_event_open(attr, -1, cpu, -1, 0)
>
> ... so this shouldn't be necessary. AFAICT, this is a SW PMU, so there
> should be no issue with using the perf_sw_context.

I might have missed it but none of the modes of invoking perf_event_open
allow monitoring a set of process, i.e., all vcpus belonging to a
particular VM, which was one of the aims and a feature I was carrying
over from the previous version. If we do not care about this...

>
> If this is a bodge to avoid opening a perf_event per vCPU thread, then I
> completely disagree with the approach. This would be better handled in
> userspace by discovering the set of threads and opening events for
> each.

... then requiring userspace to invoke perf_event_open perf vCPU will
simplify this patch.

Marc, any objections?

>
> Thanks,
> Mark.
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