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[209.51.188.17]) by mx.google.com with ESMTPS id gv25si677550ejb.209.2019.10.24.23.53.49 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Thu, 24 Oct 2019 23:53:49 -0700 (PDT) Received-SPF: pass (google.com: domain of qemu-devel-bounces+patch=linaro.org@nongnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; Authentication-Results: mx.google.com; dkim=fail header.i=@linaro.org header.s=google header.b=gCiZ80fP; spf=pass (google.com: domain of qemu-devel-bounces+patch=linaro.org@nongnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom="qemu-devel-bounces+patch=linaro.org@nongnu.org"; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=linaro.org Received: from localhost ([::1]:56100 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iNtTn-00047A-Rk for patch@linaro.org; Fri, 25 Oct 2019 02:53:47 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:35543) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iNtE1-0003Hu-Ji for qemu-devel@nongnu.org; Fri, 25 Oct 2019 02:37:31 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iNtE0-0008Qn-5r for qemu-devel@nongnu.org; Fri, 25 Oct 2019 02:37:29 -0400 Received: from mail-wr1-x42d.google.com ([2a00:1450:4864:20::42d]:46549) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1iNtDz-0008QG-Dl for qemu-devel@nongnu.org; Fri, 25 Oct 2019 02:37:28 -0400 Received: by mail-wr1-x42d.google.com with SMTP id n15so884227wrw.13 for ; Thu, 24 Oct 2019 23:37:27 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linaro.org; s=google; h=from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=9UrS1uw6AZDTMRq/tP2Ot5sPWM4GF3Yu6lTDbahkAEo=; b=gCiZ80fP6IaH1GG9J2JrpclaWG4fUpRiyvGpWRjlE6f2tZK2YkikjIjyzDdefpjyo4 1F0Z0lfOYSbWSjuaz7EvwCRcItiERn8OJmDYbYMxPBNL1nFJyEUIBIAgXGQNM4/+gkjw jhvKqFPsQQCf6qtrTLPd4+Eb+ZdMvqT7mdhin3AYzvnPg1/03xkqQZtTkzo84bodtHrn nYkfWabUTWwPP6yr4UYciTIVsqTscRpu6SNM6kZeLpcejXmpU6mtMgP/b1PLih1+DyHi 6QLrM7gNo3rVIdsxoOMfnypTppsW4Bn8bF+cWGbjWLiRuwReRFvHpgyzFB7cRiaYIVyB 6DxQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=9UrS1uw6AZDTMRq/tP2Ot5sPWM4GF3Yu6lTDbahkAEo=; b=ao8ILi/I4oGbyfjAUB8ojmH0CrcvEvXZbTrZ7Q4SHjUlY3+3SXF1E3WDweYFDXxXk9 igAwrhYnYelZDowvhO+rJ76lsbJxbGJp1OgiwGYDTakD/B/4q3SwggC+mTqkU55TNwjG N5LXnbksutG5vcm85sv3QBSAC5bH/WnXAJOxo5/+G+xGzNJ0JKkTYFeGwDOFIvFmGkfy SPNDIKbpkLNmJSupwkzX0//SEeFsHxvZ/pKj7skm0G3WBJjQKn32yRFcNiKeT8zlIiyl SkV0wgAQ3eZ0bPgk8fVPwVE33BMwD1iHtKeCzxZmC8WB96Obpg+C7NhM8bOqA6NECfjb +C3w== X-Gm-Message-State: APjAAAWbDdvfJ6q30EDCWOLhjkYvwVuU+5DfHKVlscfzAby7I/KFLH+8 yhIJJme5mzHp1ZoNQGIUF3XbgQ== X-Received: by 2002:adf:f40a:: with SMTP id g10mr1262888wro.228.1571985446192; Thu, 24 Oct 2019 23:37:26 -0700 (PDT) Received: from zen.linaroharston ([51.148.130.216]) by smtp.gmail.com with ESMTPSA id z189sm1918933wmc.25.2019.10.24.23.37.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 24 Oct 2019 23:37:24 -0700 (PDT) Received: from zen.lan (localhost [127.0.0.1]) by zen.linaroharston (Postfix) with ESMTP id A80581FFAB; Fri, 25 Oct 2019 07:37:15 +0100 (BST) From: =?utf-8?q?Alex_Benn=C3=A9e?= To: peter.maydell@linaro.org Subject: [PULL v2 21/73] docs/devel: add plugins.rst design document Date: Fri, 25 Oct 2019 07:36:21 +0100 Message-Id: <20191025063713.23374-22-alex.bennee@linaro.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20191025063713.23374-1-alex.bennee@linaro.org> References: <20191025063713.23374-1-alex.bennee@linaro.org> MIME-Version: 1.0 X-detected-operating-system: by eggs.gnu.org: Genre and OS details not recognized. X-Received-From: 2a00:1450:4864:20::42d X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Richard Henderson , =?utf-8?q?Alex_Benn?= =?utf-8?b?w6ll?= , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+patch=linaro.org@nongnu.org Sender: "Qemu-devel" This is mostly extracted from Emilio's more verbose commit comments with some additional verbiage from me. Signed-off-by: Alex Bennée Reviewed-by: Richard Henderson -- 2.20.1 diff --git a/docs/devel/index.rst b/docs/devel/index.rst index 1ec61fcfed9..2ff058bae38 100644 --- a/docs/devel/index.rst +++ b/docs/devel/index.rst @@ -22,3 +22,4 @@ Contents: decodetree secure-coding-practices tcg + plugins diff --git a/docs/devel/plugins.rst b/docs/devel/plugins.rst new file mode 100644 index 00000000000..b18fb6729e3 --- /dev/null +++ b/docs/devel/plugins.rst @@ -0,0 +1,112 @@ +.. + Copyright (C) 2017, Emilio G. Cota + Copyright (c) 2019, Linaro Limited + Written by Emilio Cota and Alex Bennée + +================ +QEMU TCG Plugins +================ + +QEMU TCG plugins provide a way for users to run experiments taking +advantage of the total system control emulation can have over a guest. +It provides a mechanism for plugins to subscribe to events during +translation and execution and optionally callback into the plugin +during these events. TCG plugins are unable to change the system state +only monitor it passively. However they can do this down to an +individual instruction granularity including potentially subscribing +to all load and store operations. + +API Stability +============= + +This is a new feature for QEMU and it does allow people to develop +out-of-tree plugins that can be dynamically linked into a running QEMU +process. However the project reserves the right to change or break the +API should it need to do so. The best way to avoid this is to submit +your plugin upstream so they can be updated if/when the API changes. + + +Exposure of QEMU internals +-------------------------- + +The plugin architecture actively avoids leaking implementation details +about how QEMU's translation works to the plugins. While there are +conceptions such as translation time and translation blocks the +details are opaque to plugins. The plugin is able to query select +details of instructions and system configuration only through the +exported *qemu_plugin* functions. The types used to describe +instructions and events are opaque to the plugins themselves. + +Usage +===== + +The QEMU binary needs to be compiled for plugin support: + +:: + configure --enable-plugins + +Once built a program can be run with multiple plugins loaded each with +their own arguments: + +:: + $QEMU $OTHER_QEMU_ARGS \ + -plugin tests/plugin/libhowvec.so,arg=inline,arg=hint \ + -plugin tests/plugin/libhotblocks.so + +Arguments are plugin specific and can be used to modify their +behaviour. In this case the howvec plugin is being asked to use inline +ops to count and break down the hint instructions by type. + +Plugin Life cycle +================= + +First the plugin is loaded and the public qemu_plugin_install function +is called. The plugin will then register callbacks for various plugin +events. Generally plugins will register a handler for the *atexit* +if they want to dump a summary of collected information once the +program/system has finished running. + +When a registered event occurs the plugin callback is invoked. The +callbacks may provide additional information. In the case of a +translation event the plugin has an option to enumerate the +instructions in a block of instructions and optionally register +callbacks to some or all instructions when they are executed. + +There is also a facility to add an inline event where code to +increment a counter can be directly inlined with the translation. +Currently only a simple increment is supported. This is not atomic so +can miss counts. If you want absolute precision you should use a +callback which can then ensure atomicity itself. + +Finally when QEMU exits all the registered *atexit* callbacks are +invoked. + +Internals +========= + +Locking +------- + +We have to ensure we cannot deadlock, particularly under MTTCG. For +this we acquire a lock when called from plugin code. We also keep the +list of callbacks under RCU so that we do not have to hold the lock +when calling the callbacks. This is also for performance, since some +callbacks (e.g. memory access callbacks) might be called very +frequently. + + * A consequence of this is that we keep our own list of CPUs, so that + we do not have to worry about locking order wrt cpu_list_lock. + * Use a recursive lock, since we can get registration calls from + callbacks. + +As a result registering/unregistering callbacks is "slow", since it +takes a lock. But this is very infrequent; we want performance when +calling (or not calling) callbacks, not when registering them. Using +RCU is great for this. + +We support the uninstallation of a plugin at any time (e.g. from +plugin callbacks). This allows plugins to remove themselves if they no +longer want to instrument the code. This operation is asynchronous +which means callbacks may still occur after the uninstall operation is +requested. The plugin isn't completely uninstalled until the safe work +has executed while all vCPUs are quiescent.