Le vendredi 12 octobre 2012 à 11:12 +0200, Yann Droneaud a écrit : > Hi, > > Le jeudi 11 octobre 2012 à 13:44 -0700, Roland Dreier a écrit : > > On Thu, Oct 11, 2012 at 8:34 AM, Animesh K Trivedi1 <[email protected]> > > wrote: > > > > > > During memory memory registration, userspace buffers also go through same > > > API calls (dma_map_sg_attrs(...)). > > > What I am confused about why no such synchronization primitives are > > > required in userspace before accessing > > > an RDMA data buffer just after when an incoming write/recv (DMA on it) is > > > finished? Who guarantees data > > > freshness? > > > > No one has really ever tried to deal with the issue of userspace RDMA on > > a cache-incoherent architecture. Basically if you try the current stack, > > the > > in-kernel users (IPoIB etc) should be OK but libibverbs etc. will be > > completely > > broken. > > > > With the current ARMv7 Cortex-A9 / Cortex-A15 MPCore and the upcoming > ARM 64 bits architecture eg ARMv8 aka Aarch64, one might want in the > near future use RDMA (InfiniBand/RoCE) with them to create highly > parallel system with low-power consumption. > > But this question and some reading about ARM memory management makes me > feel pretty unsure of the ability to use RDMA (InfiniBand) on ARM. > > In this article: "ARM, DMA, and memory management" > http://lwn.net/Articles/440221/"; > it is said that a memory page must not be mapped multiple time with > different caching attributes. > > This article take the point of Linaro's developers who want to upload > texture to the GPU without holding them in caches. This behavior might > also be applicable to RDMA as well: writing to a memory zone to be > either local IBV_WR_SEND, local IBV_WR_RDMA_WRITE or remote > IBV_WR_RDMA_READ, there's probably no need to keep it in cache. > > You could also read this other article "CMA [contiguous memory > allocator] and ARM" http://lwn.net/Articles/450286/ > and "ARM's multiply-mapped memory mess" http://lwn.net/Articles/409689/ > > After reading this, and not being an ARM expert, I'm asking myself about > a possible RDMA (InfiniBand) support on ARM. >
I've found more information in "Implementing DMA on ARM SMP Systems", Application Note 228 http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dai0228a/index.html ARMv7 Cortex-A9 seems to be a cache-coherent architecture. Regards. -- Yann Droneaud OPTEYA -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
