From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 32070 invoked by alias); 17 Jun 2008 19:37:03 -0000 Received: (qmail 32057 invoked by uid 22791); 17 Jun 2008 19:37:03 -0000 X-Spam-Check-By: sourceware.org Received: from bluesmobile.specifix.com (HELO bluesmobile.specifix.com) (216.129.118.140) by sourceware.org (qpsmtpd/0.31) with ESMTP; Tue, 17 Jun 2008 19:36:43 +0000 Received: from [127.0.0.1] (bluesmobile.specifix.com [216.129.118.140]) by bluesmobile.specifix.com (Postfix) with ESMTP id 072693BF5C; Tue, 17 Jun 2008 12:36:40 -0700 (PDT) Subject: Re: [patch] Reverse Execution in SID, Reverse Debugging with GDB and SID From: Michael Snyder To: Dave Brolley Cc: sid@sources.redhat.com, gdb@sources.redhat.com In-Reply-To: <4858013F.1090507@redhat.com> References: <450EF997.1010601@redhat.com> <4858013F.1090507@redhat.com> Content-Type: text/plain Date: Tue, 17 Jun 2008 19:37:00 -0000 Message-Id: <1213731400.3601.990.camel@localhost.localdomain> Mime-Version: 1.0 X-Mailer: Evolution 2.10.3 (2.10.3-7.fc7) Content-Transfer-Encoding: 7bit Mailing-List: contact sid-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: sid-owner@sourceware.org X-SW-Source: 2008-q2/txt/msg00003.txt.bz2 On Tue, 2008-06-17 at 14:23 -0400, Dave Brolley wrote: > Micheal Snyder is going to be doing some more work on revserse > debugging/execution and he reminded me that I never did commit this > work. It's committed now. Woo hoo, so now there is at least one target (xstormy16) where we can actually *test* reverse debugging. Should be a good step toward committing it to gdb. > Dave Brolley wrote: > > Hi, > > > > The attached patches implement > > > > 1) Infrastructure for reverse execution of in SID > > 2) Target specific implementation for xstormy16 > > > > This work is intended to be used in conjunction with Michael Snyder's > > work on reverse debugging in GDB, but I suppose that the idea of > > executing in reverse need not necessarily be restricted only to this > > purpose. > > > > I'm submitting the general infrastructure work for approval to commit. > > The xstormy16 specific implementation was done mainly as an example, > > and perhaps should also be committed. > > > > Here is an overview of the changes and how they work: > > > > o The changes the the scheduling components are mainly to have them > > drive the value of 'now' for each scheduled event. Currently they > > drive the value zero which is not used on the receiving end. This > > provides a mechanism for a scheduled component to know what 'time' > > each scheduled event occurs. Components, like memory, which are not > > scheduled, but which need to know at what 'time' an event occurred can > > query the scheduler's time attribute to obtain the same information. > > The scheduler's set_time method has also been updated so that it > > cancels all pending events. This change was probably already > > necessary, but the need was exposed during testing of this feature. > > > > o A new component class mix-in called reversible_component has been > > created to consolidate the common needs of a component which may have > > to step back in 'time'. The main features are a reversible? attribute > > which tells the component that it should be keeping track of events > > and a restore-to-time! pin which tells the component to restore it's > > state to a given 'time'. > > > > o A new utility class called change_log has been added to aid those > > components which choose to implement this using change logging > > techniques. > > > > o The basic_cpu class has been given specific knowledge of what it > > means to execute in reverse. This is controlled by it's new > > exec-direction attribute which can be set to "forward" or "backward". > > This attribute is checked when the step-insns pin is driven. If the > > direction is forward, then it's business as usual. If it's backward, > > then the cpu does what is necessary to step backward and then resets > > the scheduler to that 'time'. The scheduler in turn drives its > > time-set pin to notify other components in the system to restore > > themselves to that 'time'. Note that reverse execution need not be one > > insn at a time. I did this for the xstormy16 example so that stepi > > would work in reverse with gdb. Note that in the xstormy16 example, > > breakpoints and watchpoints are also supported while executing in > > reverse. > > > > The idea is that components in the system which have state implement > > some method of restoring their state to what it was at a given 'time'. > > Whatever makes the most sense for each particular component. The cpu > > is an obvious example as is memory. More complex systems may have > > other components with this requirement. For the xstormy16, cpu and > > memory are the only components requiring this capability. > > > > In particular, the xstormy16 cpu component needed only to track > > changes in the pc and the gr registers. Using specific knowlege of the > > kinds of changes that are possible to the pc, in particular I was able > > to implement a change logging system that uses only 1 byte for small > > pc changes (i.e. less than 128 bytes) and 3 bytes otherwise. Similarly > > for the gr registers, a 2 byte mask indicates which registers have > > changed and then only the changed registers are added to the change > > log record. Many change log records are therefore 5 bytes or less. > > > > One interesting 'feature' of the current implementation is that if a > > program has been debugged to completion and then debugging has started > > again (i.e. the gdb 'target' command establishes a new connection with > > SID), one can debug backward past the beginning of the program (with a > > warning from SID) and back into the previous execution instance. The > > feature is handy in the case that you use the GDB continue command and > > end up at the end of the program by mistake. > > > > Comments, ideas, and, of course, approval to commit the infrastructure > > patch please. I can also commit the xstormy16 specific parts if desired. > > > > Dave > >