From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:50235) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gpCDX-0005di-O7 for qemu-devel@nongnu.org; Thu, 31 Jan 2019 08:17:20 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gpCDW-00032d-RG for qemu-devel@nongnu.org; Thu, 31 Jan 2019 08:17:19 -0500 Received: from mail.ispras.ru ([83.149.199.45]:52404) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gpCDW-000326-Jm for qemu-devel@nongnu.org; Thu, 31 Jan 2019 08:17:18 -0500 From: Pavel Dovgalyuk Date: Thu, 31 Jan 2019 16:17:21 +0300 Message-ID: <20190131131721.23264.15562.stgit@pasha-VirtualBox> In-Reply-To: <20190131131520.23264.75724.stgit@pasha-VirtualBox> References: <20190131131520.23264.75724.stgit@pasha-VirtualBox> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Subject: [Qemu-devel] [PATCH v11 21/25] replay: document development rules List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: kwolf@redhat.com, peter.maydell@linaro.org, war2jordan@live.com, crosthwaite.peter@gmail.com, boost.lists@gmail.com, artem.k.pisarenko@gmail.com, quintela@redhat.com, ciro.santilli@gmail.com, jasowang@redhat.com, mst@redhat.com, armbru@redhat.com, mreitz@redhat.com, maria.klimushenkova@ispras.ru, dovgaluk@ispras.ru, kraxel@redhat.com, pavel.dovgaluk@ispras.ru, thomas.dullien@googlemail.com, pbonzini@redhat.com, alex.bennee@linaro.org, dgilbert@redhat.com, rth@twiddle.net This patch introduces docs/devel/replay.txt which describes the rules that should be followed to make virtual devices usable in record/replay mode. Signed-off-by: Pavel Dovgalyuk -- v9: fixed external virtual clock description (reported by Artem Pisarenko) --- docs/devel/replay.txt | 46 ++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 46 insertions(+) create mode 100644 docs/devel/replay.txt diff --git a/docs/devel/replay.txt b/docs/devel/replay.txt new file mode 100644 index 0000000..e641c35 --- /dev/null +++ b/docs/devel/replay.txt @@ -0,0 +1,46 @@ +Record/replay mechanism, that could be enabled through icount mode, expects +the virtual devices to satisfy the following requirements. + +The main idea behind this document is that everything that affects +the guest state during execution in icount mode should be deterministic. + +Timers +====== + +All virtual devices should use virtual clock for timers that change the guest +state. Virtual clock is deterministic, therefore such timers are deterministic +too. + +Virtual devices can also use realtime clock for the events that do not change +the guest state directly. When the clock ticking should depend on VM execution +speed, use virtual clock with EXTERNAL attribute. It is not deterministic, +but its speed depends on the guest execution. This clock is used by +the virtual devices (e.g., slirp routing device) that lie outside the +replayed guest. + +Bottom halves +============= + +Bottom half callbacks, that affect the guest state, should be invoked through +replay_bh_schedule_event or replay_bh_schedule_oneshot_event functions. +Their invocations are saved in record mode and synchronized with the existing +log in replay mode. + +Saving/restoring the VM state +============================= + +All fields in the device state structure (including virtual timers) +should be restored by loadvm to the same values they had before savevm. + +Avoid accessing other devices' state, because the order of saving/restoring +is not defined. It means that you should not call functions like +'update_irq' in post_load callback. Save everything explicitly to avoid +the dependencies that may make restoring the VM state non-deterministic. + +Stopping the VM +=============== + +Stopping the guest should not interfere with its state (with the exception +of the network connections, that could be broken by the remote timeouts). +VM can be stopped at any moment of replay by the user. Restarting the VM +after that stop should not break the replay by the unneeded guest state change.