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

[Will Deacon]

On Wed, Jan 18, 2017 at 01:01:40PM +0000, Punit Agrawal wrote:
> 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.

So it sounds like userspace needs to deal with this by attaching to the PIDs
of the vCPUs. Given that perf kvm seems to have knowledge of vCPUs, it would
be nice to know why that logic isn't reusable here. Take a look in
tools/perf/builtin-kvm.c and if it's not up to the job, then perhaps it can
be improved.

Will