From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 49607 invoked by alias); 19 Nov 2015 17:28:58 -0000 Mailing-List: contact gdb-patches-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-patches-owner@sourceware.org Received: (qmail 49593 invoked by uid 89); 19 Nov 2015 17:28:57 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-2.1 required=5.0 tests=AWL,BAYES_00,RP_MATCHES_RCVD,SPF_HELO_PASS autolearn=ham version=3.3.2 X-HELO: mx1.redhat.com Received: from mx1.redhat.com (HELO mx1.redhat.com) (209.132.183.28) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with (AES256-GCM-SHA384 encrypted) ESMTPS; Thu, 19 Nov 2015 17:28:56 +0000 Received: from int-mx14.intmail.prod.int.phx2.redhat.com (int-mx14.intmail.prod.int.phx2.redhat.com [10.5.11.27]) by mx1.redhat.com (Postfix) with ESMTPS id 8C6A642E5D0; Thu, 19 Nov 2015 17:28:55 +0000 (UTC) Received: from [127.0.0.1] (ovpn01.gateway.prod.ext.ams2.redhat.com [10.39.146.11]) by int-mx14.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with ESMTP id tAJHSsMj002216; Thu, 19 Nov 2015 12:28:54 -0500 Message-ID: <564E06D5.3020800@redhat.com> Date: Thu, 19 Nov 2015 17:28:00 -0000 From: Pedro Alves User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: "Jose E. Marchesi" CC: gdb-patches@sourceware.org Subject: Re: [PATCH] [SPARC] callfuncs.exp: avoid spurious register differences in sparc64 targets. References: <87ziyam0yh.fsf@oracle.com> <564DF643.6000407@redhat.com> <87r3jlx5y5.fsf@oracle.com> In-Reply-To: <87r3jlx5y5.fsf@oracle.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-SW-Source: 2015-11/txt/msg00408.txt.bz2 On 11/19/2015 05:19 PM, Jose E. Marchesi wrote: > > > + -re "^pstate\[ \t\]+\[^\r\n\]+\r\n" { > > + if [istarget "sparc64-*-linux-gnu"] { > > + # Filter out the pstate register, since in sparc64 > > + # targets the Linux kernel disables pstate.PEF when > > + # returning from traps, giving spurious differences. > > Isn't this a kernel bug? It sounds like it's impossible to debug FPU > code if you e.g. step over FPU instructions? > > No, it is not a kernel bug. It is a consequence of how the sparc > kernel port handles the restoring of FP registers clobbered by kernel > code. As far as I understand it: > > When an user program uses the FPU in any way (any instruction > referencing FP registers for example) a fp_disabled trap is triggered > and the kernel enables the FPU so the user program can happily continue > executing FPU instructions. > > If at some point the user program traps into the kernel (syscall, or > whatever) with the FPU activated the kernel saves whatever FP registers > it may clobber in the corresponding thread struct. Then it disables the > FPU and returns to the user program. > > Then, if the user program uses the FPU again, another fp_disabled trap > is triggered, and the kernel will both re-activate the FPU and restore > all the "dirty" FP registers that were clobbered in the previous trap. Thanks for the explanation. So until the program re-activates the FPU, when the user displays the FP registers, gdb actually shows the fpu registers as saved in the thread struct, right? Not the values clobbered by the kernel? I'd guess so, otherwise people would have noticed the breakage sooner, and assuming the kernel does use FPU instructions itself, then you'd get other spurious register differences with callfuncs.exp too. Patch is OK assuming that. Still sounds to me that it'd be better if ptrace traps left the FPU activated if it was activate on entry, on principle of minimizing program perturbation with a ptrace observer though ... Thanks, Pedro Alves