Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752219AbaJSUXm (ORCPT ); Sun, 19 Oct 2014 16:23:42 -0400 Received: from mail-la0-f48.google.com ([209.85.215.48]:58240 "EHLO mail-la0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751583AbaJSUXk (ORCPT ); Sun, 19 Oct 2014 16:23:40 -0400 MIME-Version: 1.0 In-Reply-To: References: From: Andy Lutomirski Date: Sun, 19 Oct 2014 13:23:17 -0700 Message-ID: Subject: Re: [RFC 5/5] x86,perf: Only allow rdpmc if a perf_event is mapped To: Peter Zijlstra Cc: Valdis Kletnieks , "linux-kernel@vger.kernel.org" , Paul Mackerras , Arnaldo Carvalho de Melo , Ingo Molnar , Kees Cook , Andrea Arcangeli , Erik Bosman , Andy Lutomirski Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 16, 2014 at 5:00 PM, Andy Lutomirski 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 majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/