On 18/01/17 13:01, 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. > >> 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? Not so far, but I'm curious to find out how you determine which thread is a vcpu, let alone a given vcpu. Thanks, M. -- Jazz is not dead. It just smells funny... -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
