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[209.51.188.17]) by mx.google.com with ESMTPS id i13si1938109ilm.161.2021.03.24.07.31.03 for (version=TLS1_2 cipher=ECDHE-ECDSA-CHACHA20-POLY1305 bits=256/256); Wed, 24 Mar 2021 07:31:03 -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=adnG4M9C; 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]:44316 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lP4XH-0006LL-A2 for patch@linaro.org; Wed, 24 Mar 2021 10:31:03 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:51548) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lP4Wk-0006Hx-UX for qemu-devel@nongnu.org; Wed, 24 Mar 2021 10:30:30 -0400 Received: from mail-wm1-x334.google.com ([2a00:1450:4864:20::334]:52854) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1lP4Wj-0007KC-26 for qemu-devel@nongnu.org; Wed, 24 Mar 2021 10:30:30 -0400 Received: by mail-wm1-x334.google.com with SMTP id d191so13009972wmd.2 for ; Wed, 24 Mar 2021 07:30:28 -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=kLzVba92sB3FPFp388sRp+Thx4lW+oVZeKfAa3xPR1Q=; b=adnG4M9CBykaDOtOvvr58emmLoAdPbBEwvf1Ill/6WfuUlotqVMemiATHnYKMT3Yni aVLGy0JQsv61e8RZzBZgxN9xQ5N7usT0BKe3GU4nVXk7e0mX6ixDsTFSeaABore8YC0R HVzlzprcLeZIwXySCWM4VRqP1tg7hYRawVtiH/JB2wC/kU0RhkPKujyiTonKqIMGQL6G 5X7kzbcYPAZ3qCXKq9ucUWg5roOg4+tmvy2OSurolC1uA+OK+o1lUEWqWqHUoCbsBoo6 sMAPgxfG8gEaAVNHEkOdmDs9+1O6Ud8vT3K8MX/kL6NLG6oAKoVlhcTJ1rBYgo/SIRLm wZZg== 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=kLzVba92sB3FPFp388sRp+Thx4lW+oVZeKfAa3xPR1Q=; b=Y1N8YiEFoD0YSwz4lF6BJ6rBiX628uFtnrRkhPDocU6bZLQCjhm33Gy5Tbxe//zrQ4 TUuDNjKgwRSMN1i75o3ioIXYt7dMV1U7PRAijJ6V73RSjvUvmjQi50sTuis7oixgU4k8 jZmvDFFdUwkBS1TPdSLhdZSaXcKUaCKgJEuGtdwDcdqkZKOjGvGWHteh+ERgODj/nHj1 +2w/VMQaTUNQ+FI/YgRaNqEsKAYIWwPD8yH2WrCWArIZ4Iv2xt34PwuzTvn8bHysHecN H5MbKRiU0NMyx/uVcI4tx20j+wm1tQWhKm54rGzzE24WqzM5qJ/OxmOw4gQk8R1LMe4Y XjTw== X-Gm-Message-State: AOAM530FvelZjnYdwh1WRge2tHV/6pzpAUW8t696tnr5PfZPRcpeKvzY MmlMyQ+5kDJI1ebNGWOUs0dQFw== X-Received: by 2002:a1c:7d4e:: with SMTP id y75mr3266665wmc.168.1616596227741; Wed, 24 Mar 2021 07:30:27 -0700 (PDT) Received: from zen.linaroharston ([51.148.130.216]) by smtp.gmail.com with ESMTPSA id w6sm3410048wrl.49.2021.03.24.07.30.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 24 Mar 2021 07:30:25 -0700 (PDT) Received: from zen.lan (localhost [127.0.0.1]) by zen.linaroharston (Postfix) with ESMTP id F0EDB1FF8F; Wed, 24 Mar 2021 14:30:21 +0000 (GMT) From: =?utf-8?q?Alex_Benn=C3=A9e?= To: peter.maydell@linaro.org Subject: [PULL 03/22] docs/devel: expand style section of memory management Date: Wed, 24 Mar 2021 14:30:02 +0000 Message-Id: <20210324143021.8560-4-alex.bennee@linaro.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20210324143021.8560-1-alex.bennee@linaro.org> References: <20210324143021.8560-1-alex.bennee@linaro.org> MIME-Version: 1.0 Received-SPF: pass client-ip=2a00:1450:4864:20::334; envelope-from=alex.bennee@linaro.org; helo=mail-wm1-x334.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=ham autolearn_force=no X-Spam_action: no action 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 aims to provide a bit more guidance for those who take on one of our "clean up memory allocation" bite-sized tasks. Signed-off-by: Alex Bennée Reviewed-by: Richard Henderson Message-Id: <20210323165308.15244-4-alex.bennee@linaro.org> -- 2.20.1 diff --git a/docs/devel/style.rst b/docs/devel/style.rst index 8b0bdb3570..260e3263fa 100644 --- a/docs/devel/style.rst +++ b/docs/devel/style.rst @@ -385,17 +385,37 @@ avoided. Low level memory management =========================== -Use of the malloc/free/realloc/calloc/valloc/memalign/posix_memalign +Use of the ``malloc/free/realloc/calloc/valloc/memalign/posix_memalign`` APIs is not allowed in the QEMU codebase. Instead of these routines, -use the GLib memory allocation routines g_malloc/g_malloc0/g_new/ -g_new0/g_realloc/g_free or QEMU's qemu_memalign/qemu_blockalign/qemu_vfree -APIs. - -Please note that g_malloc will exit on allocation failure, so there -is no need to test for failure (as you would have to with malloc). -Calling g_malloc with a zero size is valid and will return NULL. - -Prefer g_new(T, n) instead of g_malloc(sizeof(T) ``*`` n) for the following +use the GLib memory allocation routines +``g_malloc/g_malloc0/g_new/g_new0/g_realloc/g_free`` +or QEMU's ``qemu_memalign/qemu_blockalign/qemu_vfree`` APIs. + +Please note that ``g_malloc`` will exit on allocation failure, so +there is no need to test for failure (as you would have to with +``malloc``). Generally using ``g_malloc`` on start-up is fine as the +result of a failure to allocate memory is going to be a fatal exit +anyway. There may be some start-up cases where failing is unreasonable +(for example speculatively loading a large debug symbol table). + +Care should be taken to avoid introducing places where the guest could +trigger an exit by causing a large allocation. For small allocations, +of the order of 4k, a failure to allocate is likely indicative of an +overloaded host and allowing ``g_malloc`` to ``exit`` is a reasonable +approach. However for larger allocations where we could realistically +fall-back to a smaller one if need be we should use functions like +``g_try_new`` and check the result. For example this is valid approach +for a time/space trade-off like ``tlb_mmu_resize_locked`` in the +SoftMMU TLB code. + +If the lifetime of the allocation is within the function and there are +multiple exist paths you can also improve the readability of the code +by using ``g_autofree`` and related annotations. See :ref:`autofree-ref` +for more details. + +Calling ``g_malloc`` with a zero size is valid and will return NULL. + +Prefer ``g_new(T, n)`` instead of ``g_malloc(sizeof(T) * n)`` for the following reasons: * It catches multiplication overflowing size_t; @@ -409,8 +429,8 @@ Declarations like are acceptable, though. -Memory allocated by qemu_memalign or qemu_blockalign must be freed with -qemu_vfree, since breaking this will cause problems on Win32. +Memory allocated by ``qemu_memalign`` or ``qemu_blockalign`` must be freed with +``qemu_vfree``, since breaking this will cause problems on Win32. String manipulation =================== @@ -485,6 +505,8 @@ In addition, QEMU assumes that the compiler does not use the latitude given in C99 and C11 to treat aspects of signed '<<' as undefined, as documented in the GNU Compiler Collection manual starting at version 4.0. +.. _autofree-ref: + Automatic memory deallocation =============================