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Sat, 20 Mar 2021 06:37:11 -0700 (PDT) Received: from zen.linaroharston ([51.148.130.216]) by smtp.gmail.com with ESMTPSA id z9sm6395435edr.75.2021.03.20.06.37.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 20 Mar 2021 06:37:09 -0700 (PDT) Received: from zen.lan (localhost [127.0.0.1]) by zen.linaroharston (Postfix) with ESMTP id 0010D1FF8F; Sat, 20 Mar 2021 13:37:06 +0000 (GMT) From: =?UTF-8?q?Alex=20Benn=C3=A9e?= To: qemu-devel@nongnu.org Subject: [PATCH v1 03/14] docs/devel: expand style section of memory management Date: Sat, 20 Mar 2021 13:36:55 +0000 Message-Id: <20210320133706.21475-4-alex.bennee@linaro.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20210320133706.21475-1-alex.bennee@linaro.org> References: <20210320133706.21475-1-alex.bennee@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=2a00:1450:4864:20::535; envelope-from=alex.bennee@linaro.org; helo=mail-ed1-x535.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: =?UTF-8?q?Alex=20Benn=C3=A9e?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.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 --- docs/devel/style.rst | 46 ++++++++++++++++++++++++++++++++------------ 1 file changed, 34 insertions(+), 12 deletions(-) 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 ============================= -- 2.20.1