On Thu, Oct 16, 2014 at 5:00 PM, Andy Lutomirski <l...@amacapital.net> wrote: > The current cap_user_rdpmc code seems rather confused to me. On x86, > *all* events set cap_user_rdpmc if the global rdpmc control is set. > But only x86_pmu events define .event_idx, so amd uncore events won't > actually expose their rdpmc index to userspace. > > Would it make more sense to add a flag PERF_X86_EVENT_RDPMC_PERMITTED > that gets set on all events created while rdpmc == 1, to change > x86_pmu_event_idx to do something like: > > if (event->hw.flags & PERF_X86_EVENT_RDPMC_PERMITTED) > return event->hw.event_base_rdpmc + 1; > else > return 0; > > and to change arch_perf_update_userpage cap_user_rdpmc to match > PERF_X86_EVENT_RDPMC_PERMITTED? > > Then we could ditch the static key and greatly simplify writes to the > rdpmc flag by just counting PERF_X86_EVENT_RDPMC_PERMITTED events. > > This would be a user-visible change on AMD, and I can't test it. > > > On a semi-related note: would this all be nicer if there were vdso > function __u64 __vdso_perf_event__read_count(int fd, void *userpage)? > This is very easy to do nowadays. If we got *really* fancy, it would > be possible to have an rdpmc_safe in the vdso, which has some > benefits, although it would be a bit evil and wouldn't work if > userspace tracers like pin are in use. >
Also, I don't understand the purpose of cap_user_time. Wouldn't it be easier to just record the last CLOCK_MONOTONIC time and let the user call __vdso_clock_gettime if they need an updated time? --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
