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docs/devel: expand style section of memory management 

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 <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210323165308.15244-4-alex.bennee@linaro.org>
This commit is contained in:
Alex Bennée 2021-03-23 16:52:49 +00:00
parent ca955bd726
commit 9fed69e1f6
1 changed files with 32 additions and 10 deletions
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42
docs/devel/style.rst
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@ -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.
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.
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).
Prefer g_new(T, n) instead of g_malloc(sizeof(T) ``*`` n) for the following
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
=============================