The choice between rte_memcpy() and memcpy() was not documented, which has led to rte_memcpy() being used in places where it gives no benefit, such as the control path and copies of a constant size.
Document the guidance: structure assignment for fixed size objects, memcpy() for the control path and for constant sizes, and rte_memcpy() only for variable size copies in the data path. Suggested-by: Morten Brørup <[email protected]> Signed-off-by: Stephen Hemminger <[email protected]> --- lib/eal/include/generic/rte_memcpy.h | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/lib/eal/include/generic/rte_memcpy.h b/lib/eal/include/generic/rte_memcpy.h index da53b72ca8..e409c9bccc 100644 --- a/lib/eal/include/generic/rte_memcpy.h +++ b/lib/eal/include/generic/rte_memcpy.h @@ -96,6 +96,24 @@ rte_mov256(uint8_t *dst, const uint8_t *src); /** * Copy bytes from one location to another. The locations must not overlap. * + * This function exists because on some platforms the libc memcpy() + * does not generate optimal code for the copy sizes and alignments + * common in packet processing. It is not a general replacement for + * memcpy(); prefer the following, in order: + * + * - Use structure assignment when copying a fixed size object. + * The compiler generates the best possible code, and the types + * are checked. + * - Use memcpy() everywhere else, in particular in the control path, + * and whenever the size is a compile time constant. For a constant + * size the compiler inlines the copy, so rte_memcpy() gains nothing, + * and memcpy() is understood by analysis tools such as the address + * sanitizer, valgrind and _FORTIFY_SOURCE which can then detect + * overlapping or out of bounds copies. + * - Use rte_memcpy() only in the data path, and only where the size is + * variable at run time. This is the case for which it was written, + * and where it can still be faster than the libc implementation. + * * @note This is implemented as a macro, so it's address should not be taken * and care is needed as parameter expressions may be evaluated multiple times. * -- 2.53.0
