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 669203853800 for ; Mon, 17 May 2021 17:34:08 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 669203853800 Received: from fencepost.gnu.org ([2001:470:142:3::e]:53658) by eggs.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lih82-0005FG-GC for gdb@sourceware.org; Mon, 17 May 2021 13:34:06 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:39292) by fencepost.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lih80-0005em-SO for gdb@gnu.org; Mon, 17 May 2021 13:34:05 -0400 Received: from mail-wr1-x436.google.com ([2a00:1450:4864:20::436]:44733) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1lih7u-0005CI-KB for gdb@gnu.org; Mon, 17 May 2021 13:34:04 -0400 Received: by mail-wr1-x436.google.com with SMTP id i17so7262246wrq.11 for ; Mon, 17 May 2021 10:33:56 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:references:user-agent:from:to:cc:subject:date :in-reply-to:message-id:mime-version:content-transfer-encoding; bh=FRN5NxbdVzdG4FkLQJDpZhdofz87tsYGeQhczznfpjM=; b=n2s9WSqmMgSHFyhr/kH/DLxkTxtHvLYvqcHYHaS9yYjOFaMeyGF9o7w+X3ewLvj0eC GRJHfsSLHMkFx++mWauDBxV57K5ulbIwYREJsaLOpRpf+InO9tiPlTZOzo9N3E2EvIBr m425FiAal5oNCdqY5Z4uewVUT1F0Ile6ubZyK2IG1ls5ON6AKY7YjevlsuvmMixf6126 Rmm57zPE7QnsjJd7DBiBmQhwRTCgHfUnjWRB6hdafDyFQ6Cd/SswzJbJah9/PNU5WH1r qox2QIw4eqN1pUJAybPt6ZF+Lne9+Z8aOIlKlcDGTSQgo8Q/NlWDuOjp3NcqNjJYVQIL 9cRA== X-Gm-Message-State: AOAM531o3cL1FMo2FIiBF5nkwOn9nBCC0PKc1jk9fu3zXxqofpYlya3E Qiurv+aDybysD3ncna797gjMF11arc54Dg== X-Google-Smtp-Source: ABdhPJzWvTjBvBU1ZdvcrckUSWVhw1RQvbZ7GiRjcS+Zk09s+QDP53D8ItjsDarglYJ8L9jft5lAIw== X-Received: by 2002:a5d:4536:: with SMTP id j22mr918289wra.329.1621272833883; Mon, 17 May 2021 10:33:53 -0700 (PDT) Received: from zen.linaroharston ([51.148.130.216]) by smtp.gmail.com with ESMTPSA id c22sm15458574wmb.10.2021.05.17.10.33.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 17 May 2021 10:33:53 -0700 (PDT) Received: from zen (localhost [127.0.0.1]) by zen.linaroharston (Postfix) with ESMTP id 64E2D1FF7E; Mon, 17 May 2021 18:33:52 +0100 (BST) References: <87y2chjmsf.fsf@linaro.org> <6c8845b7-cc60-c8ba-3ada-6d0c6e65d8a5@linaro.org> User-agent: mu4e 1.5.13; emacs 28.0.50 From: Alex =?utf-8?Q?Benn=C3=A9e?= To: Luis Machado Cc: gdb@gnu.org, QEMU Developers , Pavel Dovgalyuk , =?utf-8?Q?Daniel_P=2E_Berrang=C3=A9?= Subject: Re: Best approach for supporting snapshots for QEMU's gdbstub? Date: Mon, 17 May 2021 18:27:06 +0100 In-reply-to: <6c8845b7-cc60-c8ba-3ada-6d0c6e65d8a5@linaro.org> Message-ID: <87bl99e03j.fsf@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Received-SPF: pass client-ip=2a00:1450:4864:20::436; envelope-from=alex.bennee@linaro.org; helo=mail-wr1-x436.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, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable 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, 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 17:34:10 -0000 Luis Machado writes: > Hi, > > On 5/14/21 1:06 PM, Alex Benn=C3=A9e 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? >>=20 > > 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. Out of interest how is rr implemented? It presents a gdb interface so I thought it was some implemented using some remote magic. >> We could of course just add a custom monitor command like the >> qemu.sstep=3D 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. 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. > >> Any thoughts/suggestions? >>=20 --=20 Alex Benn=C3=A9e