From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from eggs.gnu.org (eggs.gnu.org [IPv6:2001:470:142:3::10]) by sourceware.org (Postfix) with ESMTPS id 94FE33853809 for ; Mon, 17 May 2021 17:54:22 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 94FE33853809 Received: from fencepost.gnu.org ([2001:470:142:3::e]:54360) by eggs.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lihRe-0004h7-5r for gdb@sourceware.org; Mon, 17 May 2021 13:54:22 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:44270) by fencepost.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lihRd-0007Oc-Tk for gdb@gnu.org; Mon, 17 May 2021 13:54:21 -0400 Received: from mail-qv1-xf2e.google.com ([2607:f8b0:4864:20::f2e]:40659) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1lihRc-0004fC-1P for gdb@gnu.org; Mon, 17 May 2021 13:54:21 -0400 Received: by mail-qv1-xf2e.google.com with SMTP id e8so297277qvp.7 for ; Mon, 17 May 2021 10:54:19 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=iCdBGFtXsbMCQlT9EXnMyoD9HU/1ZTP0N3vWA2rRvh4=; b=uUJBGQHlKsn1+3kGG8tcLoSV3+pgwtxsUO4XgEZvyFCxlCNmjUTaxJ5psvtstFli4l hPcQgij5/NEuOChmNRRoaGnWovQ2DM7DZrwwm7Y2cHqHCZFh6R1P9JfbHOHFqq7imoG+ JBe4UdsYQBbzLMz4CW9taFMQgVVQi6efwBch9qgQ3LyoN8Unh3w6blX2/5iNU+J/1z9D PnNfGRZlQtPOcfkhY/zhGJjKbNxp+KSpKn5zCBc4XvRRx4SScM1Czqtr0b4nagHLovgM KzQOgyeRLWv4rrx454504RqsTKa7n9T9ZtqoX9MaqTqASyIY9KagFCExmcBNfN3dIq4/ EmmA== X-Gm-Message-State: AOAM532lyrDKSHc/C0Evl0QHx2vUaPQgBlsH3Xkm8vkVWS1SODwU7++d Y7cWmjPkWtU3mHKwcd7oxsD8gQ== X-Google-Smtp-Source: ABdhPJwUSD15Oi99BOVCx122jT0JmTEAK72WO3buPn+yowxBpf+xGOiNwPUT5173HMGHnrx8FqetuQ== X-Received: by 2002:a05:6214:a43:: with SMTP id ee3mr1165927qvb.61.1621274058829; Mon, 17 May 2021 10:54:18 -0700 (PDT) Received: from ?IPv6:2804:7f0:4841:40ad:6091:b08d:7dd2:84b1? ([2804:7f0:4841:40ad:6091:b08d:7dd2:84b1]) by smtp.gmail.com with ESMTPSA id 25sm976299qtd.51.2021.05.17.10.54.16 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 17 May 2021 10:54:18 -0700 (PDT) Subject: Re: Best approach for supporting snapshots for QEMU's gdbstub? To: =?UTF-8?Q?Alex_Benn=c3=a9e?= Cc: gdb@gnu.org, QEMU Developers , Pavel Dovgalyuk , =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= References: <87y2chjmsf.fsf@linaro.org> <6c8845b7-cc60-c8ba-3ada-6d0c6e65d8a5@linaro.org> <87bl99e03j.fsf@linaro.org> From: Luis Machado Message-ID: Date: Mon, 17 May 2021 14:54:15 -0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: <87bl99e03j.fsf@linaro.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=2607:f8b0:4864:20::f2e; envelope-from=luis.machado@linaro.org; helo=mail-qv1-xf2e.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-Spam-Status: No, score=-5.1 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, NICE_REPLY_A, SPF_HELO_PASS, SPF_SOFTFAIL, TXREP autolearn=no autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: gdb@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gdb mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 17 May 2021 17:54:25 -0000 On 5/17/21 2:27 PM, Alex Bennée wrote: > > Luis Machado writes: > >> Hi, >> >> On 5/14/21 1:06 PM, Alex Bennée wrote: >>> Hi, >>> I've been playing around with QEMU's reverse debugging support which >>> I have working with Pavel's latest patches for supporting virtio with >>> record/replay. Once you get the right command line it works well enough >>> although currently each step backwards requires replaying the entire >>> execution history until you get to the right point. >>> QEMU can quite easily snapshot the entire VM state so I was looking >>> to >>> see what the best way to integrate this would be. As far as I can tell >>> there are two interfaces gdb supports: bookmarks and checkpoints. >>> As far as I can tell bookmarks where added as part of GDB's reverse >>> debugging support but attempting to use them from the gdbstub reports: >>> (gdb) bookmark >>> You can't do that when your target is `remote' >>> so I guess that would need an extension to the stub protocol to >>> support? >>> >> >> Right. We don't support reverse step/next/continue for remote targets. >> I think this would be the most appropriate way to implement this >> feature in GDB. But it is not trivial. > > You do because ";ReverseStep+;ReverseContinue+" is part of the gdbstub > negotiation handshake. Interesting... I was looking at the vCont; packets for inferior movement. The regular c/C/s/S packet are deprecated and vCont; equivalents should be used instead. It seems the reverse continue (bc) and reverse step (bs) packets can be used, but they are not vCont packets. That's confusing. I suppose nobody took the time to implement bc/bs equivalents for vCont. > > Out of interest how is rr implemented? It presents a gdb interface so I > thought it was some implemented using some remote magic. I don't know. I have never used rr. > > > >>> We could of course just add a custom monitor command like the >>> qemu.sstep= command which could be used manually. However that would be >>> a QEMU gdbstub specific approach. >> >> That would be an easy and quick way to allow GDB to control things in >> QEMU, but I wouldn't say it is the best. Monitor commands are >> basically a bypass of the RSP where GDB sends/receives commands >> to/from the remote target. > > We have some underlying commands we can set via the monitor including: > > monitor info replay > monitor replay_seek > monitor replay_break > >> >>> The other thing would be to be more intelligent on QEMU's side and >>> save >>> snapshots each time we hit an event, for example each time we hit a >>> given breakpoint. However I do worry that might lead to snapshots >>> growing quite quickly. >> >> GDB would need to be aware of such snapshots for them to be useful. >> Otherwise GDB wouldn't be able to use them to restore state. > > What does GDB need to know about them? Does it include something like > the icount at a particular point. GDB needs to know they exist so the user can choose to go back to such snapshots. I haven't dealt with remote reverse execution implementations, but if this information can be exposed to > > I'm curious at how a break and reverse-continue is meant to work if that > breakpoint is hit multiple times from the start of a run. You need to > know if the last time you hit a particular breakpoint was in fact the > last time before where the user was when they hit reverse-continue. When you have record/replay on, there is no real "continue". GDB will instruction-step everything and will record register values and memory changes. When you reverse instruction-step, GDB will restore the state for the previous snapshot. When you reverse continue, GDB will do the same and will move the state backwards snapshot by snapshot. It is not very efficient. So, in that sense, GDB will hit all of the breakpoints again. It doesn't keep track of how many times the breakpoint was hit. It only keeps track of how many instructions were recorded and what register/memory changes happened. If you hit an instruction that GDB doesn't know how to calculate register/memory changes for, it will stop dead on its tracks. In that sense, it is also not very easy to maintain and takes a lot of instruction-parsing to work correctly. No wonder there are more performatic solutions out there. :-) > >> >>> Any thoughts/suggestions? >>> > >