From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753163Ab2JVLbz (ORCPT ); Mon, 22 Oct 2012 07:31:55 -0400 Received: from mail-lb0-f174.google.com ([209.85.217.174]:61255 "EHLO mail-lb0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751438Ab2JVLby (ORCPT ); Mon, 22 Oct 2012 07:31:54 -0400 MIME-Version: 1.0 In-Reply-To: <20121021180341.GA4128@gmail.com> References: <1350658326-14715-1-git-send-email-eranian@google.com> <1350658326-14715-3-git-send-email-eranian@google.com> <1350664077.2768.32.camel@twins> <1350665152.2768.41.camel@twins> <20121019172018.GA2095@tassilo.jf.intel.com> <20121021165523.GA29247@gmail.com> <20121021180341.GA4128@gmail.com> Date: Mon, 22 Oct 2012 13:31:45 +0200 Message-ID: Subject: Re: [PATCH 2/2] perf: SNB exclusive PMU access for INST_RETIRED:PREC_DIST From: Stephane Eranian To: Ingo Molnar Cc: Andi Kleen , Peter Zijlstra , LKML , "mingo@elte.hu" , Arnaldo Carvalho de Melo , Jiri Olsa Content-Type: text/plain; charset=UTF-8 X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Oct 21, 2012 at 8:03 PM, Ingo Molnar wrote: > > * Stephane Eranian wrote: > >> On Sun, Oct 21, 2012 at 6:55 PM, Ingo Molnar wrote: >> > >> > * Andi Kleen wrote: >> > >> >> > > > This isn't limited to admin, right? So the above turns into a DoS on the >> >> > > > console. >> >> > > > >> >> > > Ok, so how about a WARN_ON_ONCE() instead? >> >> > >> >> > That should be fine I guess ;-) >> >> >> >> imho there is need for a generic mechanism to return an error >> >> string to the user program without such hacks. >> > >> > Agreed - we could return an 'extended errno' long error return >> > value, which in reality is a pointer to an error string (in >> > perf_attr::error_str), and copy that string to user-space at >> > perf syscall return time. >> > >> I assume by perf_attr:error_str, you actually mean: >> >> struct perf_event_attr { >> char error_str[PERF_ERR_LEN]; >> }; >> >> Right? > > I don't think we should allocate space in the attr, instead we > should use something like: > > u8 __user *err_str; > u32 err_str_len; > > which would be filled in by tooling with a string and a max_len > value, and strncpy_to_user() could do the rest on the kernel > side. [ A minor complication is that we don't have a > strncpy_to_user() method at the moment. ] > > Static strings could be handled this way. > > [ Dynamic strings could be supported too with a few tricks, > although I doubt it matters in practice. ] > Ok, but this still limits returning error string to the perf_event_open() syscall, not read(), ioctl() and such. I am fine with this change. However, I think it should be added separately from my inst_retired:prec_dist patch. It has a broader impact. >> > Thus error-string aware tooling could print the error string. >> > >> > So PMU drivers could do something obvious like: >> > >> > return (long)"perf: INST_RETIRED.PREC_DIST only works in exclusive mode"; >> > >> > The perf syscall notices these pointers by noticing that the >> > error code returned is outside the errno range. >> >> Is that always the case on all archs? > > I think yes - and if not then it can be solved via some trivial > offset value added to it on such an architecture, without > complicating the code on normal architectures. > > Thanks, > > Ingo