Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932476Ab2JUSDr (ORCPT ); Sun, 21 Oct 2012 14:03:47 -0400 Received: from mail-we0-f174.google.com ([74.125.82.174]:54685 "EHLO mail-we0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932420Ab2JUSDq (ORCPT ); Sun, 21 Oct 2012 14:03:46 -0400 Date: Sun, 21 Oct 2012 20:03:41 +0200 From: Ingo Molnar To: Stephane Eranian Cc: Andi Kleen , Peter Zijlstra , LKML , "mingo@elte.hu" , Arnaldo Carvalho de Melo , Jiri Olsa Subject: Re: [PATCH 2/2] perf: SNB exclusive PMU access for INST_RETIRED:PREC_DIST Message-ID: <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2199 Lines: 69 * 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. ] > > 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 -- 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/