<snip> > > > From: Honnappa Nagarahalli [mailto:honnappa.nagaraha...@arm.com] > > Sent: Wednesday, 27 July 2022 19.38 > > > > [...] > > > > > > > > > Yes, x86 needs 16B alignment for NT load/stores But that's > > supposed > > > > to be arch > > > > > specific limitation, that we probably want to hide, no? > > > > > > Correct. However, optional hints for optimization purposes will be > > available. > > > And it is up to the architecture specific implementation to make the > > best use > > > of these hints, or just ignore them. > > > > > > > > Inside the function can check alignment of both src and dst and > > > > decide should it > > > > > use NT load/store instructions or just do normal copy. > > > > IMO, the normal copy should not be done by this API under any > > > > conditions. Why not let the application call memcpy/rte_memcpy > > > > when the NT copy is not applicable? It helps the programmer to > > understand > > > > and debug the issues much easier. > > > > > > Yes, the programmer must choose between normal memcpy() and non- > > > temporal rte_memcpy_nt(). I am offering new functions, not modifying > > > memcpy() or rte_memcpy(). > > > > > > And rte_memcpy_nt() will silently fall back to normal memcpy() if > > non- > > > temporal copying is unavailable, e.g. on POWER and RISC-V > > architectures, > > > which don't have NT load/store instructions. > > I am talking about a scenario where the application is being ported > > between architectures. Not everyone knows about the capabilities of > > the architecture. It is better to indicate upfront (ex: compilation > > failures) that a certain feature is not supported on the target > > architecture rather than the user having to discover through painful > > debugging. > > I'm considering rte_memcpy_nt() a performance optimized variant of > memcpy(), where the performance gain is less cache pollution. Thus, silent > fallback to memcpy() should suffice. > > Other architecture differences also affect DPDK performance; the inability to > perform non-temporal load/store just one more to the (undocumented) list. > > Failing at build time if NT load/store is unavailable by the architecture > would > prevent the function from being used by other DPDK libraries, e.g. by the > rte_pktmbuf_copy() function used by the pdump library. The other libraries in DPDK need to provide NT versions as the libraries need to cater for not-NT use cases as well. i.e. we cannot hide a NT copy under rte_pktmbuf_copy() API, we need to have rte_pktmbuf_copy_nt()
> > I don't oppose to your idea, I just don't have any idea how to reasonably > implement it. So I'm trying to defend why it is not important. I am suggesting that the applications could implement #ifdef depending on the architecture.
