From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 31309 invoked by alias); 21 Aug 2009 07:48:43 -0000 Received: (qmail 31296 invoked by uid 22791); 21 Aug 2009 07:48:38 -0000 X-SWARE-Spam-Status: No, hits=-3.1 required=5.0 tests=AWL,BAYES_00,SPF_HELO_PASS,SPF_PASS X-Spam-Check-By: sourceware.org Received: from lo.gmane.org (HELO lo.gmane.org) (80.91.229.12) by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Fri, 21 Aug 2009 07:48:33 +0000 Received: from list by lo.gmane.org with local (Exim 4.50) id 1MeOrO-0004po-BX for gdb@sources.redhat.com; Fri, 21 Aug 2009 09:48:30 +0200 Received: from 82-68-48-14.dsl.in-addr.zen.co.uk ([82.68.48.14]) by main.gmane.org with esmtp (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Fri, 21 Aug 2009 09:48:30 +0200 Received: from jsmith by 82-68-48-14.dsl.in-addr.zen.co.uk with local (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Fri, 21 Aug 2009 09:48:30 +0200 To: gdb@sources.redhat.com From: Julian Smith Subject: Re: Some questions about gdb's remote protocol and reverse debugging Date: Fri, 21 Aug 2009 10:57:00 -0000 Message-ID: <20090821084807.5e296baf.jsmith@undo-software.com> References: <20090811224401.4d9e8942.jsmith@undo-software.com> <4A81FD36.2040009@vmware.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-IsSubscribed: yes Mailing-List: contact gdb-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-owner@sourceware.org X-SW-Source: 2009-08/txt/msg00205.txt.bz2 On Tue, 11 Aug 2009 16:22:30 -0700 Michael Snyder wrote: > Julian Smith wrote: > > Hello > > > > I've been trying out gdb-cvs's remote protocol, specifically the > > commands for reverse debugging, and i have a couple of questions that i > > was hoping someone might be able to help me with. I'm using Linux on > > x86-32 and x86-64. > > Cool, welcome! Join the fun. > > > First, if i'm understanding things correctly, gdb appears to default to > > software breakpoints, using the 'Z0' and 'z0' commands and, if these > > aren't supported by the remote target, it then tries to use 'M' and 'm' > > to write breakpoints directly into the inferior's memory. > > That's right... > > > Is there any way to tell gdb to try to use hardware breakpoints (with > > the 'Z1' and 'z1' commands) before resorting to 'M' and 'm' ? [In the > > environment i'm working in, UndoDB, hardware breakpoints are more > > convenient because they don't require any patching up of %pc, and > > poking breakpoints directly into memory is not supported.] > > Not as such, no. You use a different syntax to set a hardware > breakpoint. The command is "hbreak" instead of "break". You > should at least be able to try stuff out with that. > > You may need to enable the Z1 packet, like this (I'm not sure > if it's enabled by default): > > set remote haredware-breakpoint-packet 1 Ah, i hadn't noticed this command, though i have been using `hbreak'. Thanks. I'm also trying out disabling the software-breakpoint packet, and things seem to be working better now. > > > Second, am i right in thinking that gdb does things like reverse-step > > and reverse-next by effectively doing many reverse-stepi's (with 'bs'), > > interleaved with 'g' commands to get the registers? If so, are there > > any plans to try to avoid the overhead of this somehow ? > > There are only the two actual reverse-execution packets -- 'bs' and > 'bc'. Gdb uses them in exactly the same contexts that it would use > 's' and 'c' if going forward. As such, "step" is always going to be > implemented as one or more 's' requests, whereas "next" may involve > a mixture of 's' and 'c' requests. The same is true going backward. Ok. But... isn't there always going to be a difference when going backwards, e.g. if an instruction jumps to an address in a register, one cannot know about this jump when stepping backwards (and so, for example, one cannot first go back to the destination of the jump, do a `bs' to step back over the jump, then carry on with `bc')? Perhaps this sort of indirect jump never occurs within a function though, so is not a problem for things like reverse-step. Anyway, many thanks for you help, 'tis much appreciated. I'm still tinkering with things, but i think i should be able to make progress now. Cheers, - Julian -- http://undo-software.com/