Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752629AbaJQABT (ORCPT ); Thu, 16 Oct 2014 20:01:19 -0400 Received: from mail-lb0-f177.google.com ([209.85.217.177]:50926 "EHLO mail-lb0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751033AbaJQABS (ORCPT ); Thu, 16 Oct 2014 20:01:18 -0400 MIME-Version: 1.0 In-Reply-To: References: From: Andy Lutomirski Date: Thu, 16 Oct 2014 17:00:56 -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 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. --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/