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[209.51.188.17]) by mx.google.com with ESMTPS id x10si17305792qkl.274.2019.10.14.03.53.28 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 14 Oct 2019 03:53:29 -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=vyFO8v3O; 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]:46988 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iJxyi-0002bj-JO for patch@linaro.org; Mon, 14 Oct 2019 06:53:28 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:44498) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iJxvM-0007K6-2d for qemu-devel@nongnu.org; Mon, 14 Oct 2019 06:50:02 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iJxvJ-00062X-Hz for qemu-devel@nongnu.org; Mon, 14 Oct 2019 06:49:59 -0400 Received: from mail-wr1-x42c.google.com ([2a00:1450:4864:20::42c]:36424) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1iJxvJ-0005xx-6T for qemu-devel@nongnu.org; Mon, 14 Oct 2019 06:49:57 -0400 Received: by mail-wr1-x42c.google.com with SMTP id y19so19160045wrd.3 for ; Mon, 14 Oct 2019 03:49:56 -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=Yb3UF5h0z67h7aWNI1Qg9AnW+g8R23U+pnxlZeqlysw=; b=vyFO8v3OS/k8Wf471kD3TJnpLGb4zfQWiuYXX+4soW41jv5gFC0KeY0wb7QpqUE0Wd CDZP3fgp+qhLvlcCjG9YopoDz2ro35pMwl9rVr0nXFvefiBam/rmmbqvrXWJoE4+cIPC S8G8eENKH2EWi/Tqi3Y9QS5/rCJUnxqllfIyJO6h/vaETbXpgHekccA8npm0zodsnSOy GEWhpmDkgRvuxNXLi+m5JDm4aPCzDzoNNP9mxEQADmAGweI92Lu8BtHYkattV06O1pPe sCRc16MQB1Lpg2+aQ9SDUaT0dD3roxmzrO59K5WPPAPPgi2w8ulxy/JDJsEk4GE5Poe+ 94ng== 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=Yb3UF5h0z67h7aWNI1Qg9AnW+g8R23U+pnxlZeqlysw=; b=fkl07Fdk5oQkjmi41ZdQR5nsiPzNhpcQohQcZ3nRmM6Y+BrMQfG5158KHcX+TooNu9 b+sjksW+fIN9hKTYi5a/L+jybbu1mJpsv6sRxzYiJR6exVfpIUW/oJpR4TZsfLVJ05gE NXfYQBzNGu6TNuj0BvJpjDpG9vhprL4ZcH8npyeCNkPv5EjmcjSfZjUJFVdqMWGP/hPj xnTGGw7XonznGmJ2pR6XlcnFi5TIpDCYRq/U6W/y8SoCWgeWMqQ4+gksUB1Mlv01fx2D jz+93bPHJGaWUlEwhseQKWbrV39gX9OwF+PLgtSULVq2xSD4tzuu2cKPNHbSauRReKEf /u7w== X-Gm-Message-State: APjAAAXtyggiXnvgbJjvkTbOfMONV3yl+/2stWmpOCopidZ5ZrXWoLjK FH9Vkyh+baE1Q24lz+W4KN68DA== X-Received: by 2002:a05:6000:12d1:: with SMTP id l17mr14601892wrx.2.1571050194179; Mon, 14 Oct 2019 03:49:54 -0700 (PDT) Received: from zen.linaroharston ([51.148.130.216]) by smtp.gmail.com with ESMTPSA id p5sm24112231wmi.4.2019.10.14.03.49.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 14 Oct 2019 03:49:51 -0700 (PDT) Received: from zen.lan (localhost [127.0.0.1]) by zen.linaroharston (Postfix) with ESMTP id 1B04B1FF92; Mon, 14 Oct 2019 11:49:50 +0100 (BST) From: =?utf-8?q?Alex_Benn=C3=A9e?= To: qemu-devel@nongnu.org Subject: [PATCH v5 05/55] docs/devel: add plugins.rst design document Date: Mon, 14 Oct 2019 11:48:58 +0100 Message-Id: <20191014104948.4291-6-alex.bennee@linaro.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20191014104948.4291-1-alex.bennee@linaro.org> References: <20191014104948.4291-1-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::42c 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: robert.foley@futurewei.com, Richard Henderson , peter.puhov@futurewei.com, aaron@os.amperecomputing.com, cota@braap.org, =?utf-8?q?Alex_Benn?= =?utf-8?b?w6ll?= 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 --- v4 - some rewording and tweaks - made non-atomicity of inline ops more explicit - expanded description of plugin unload v5 - more explicit statements: - cannot modify system state, only observer - call to action to upstream plugins --- docs/devel/index.rst | 1 + docs/devel/plugins.rst | 112 +++++++++++++++++++++++++++++++++++++++++ 2 files changed, 113 insertions(+) create mode 100644 docs/devel/plugins.rst -- 2.20.1 diff --git a/docs/devel/index.rst b/docs/devel/index.rst index 1ec61fcfed..2ff058bae3 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 0000000000..b18fb6729e --- /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.