Thursday, February 14, 2019 12:19 PM, Burakov, Anatoly: > Subject: Re: [dpdk-dev] [PATCH 0/6] introduce DMA memory mapping for > external memory > > On 13-Feb-19 7:24 PM, Shahaf Shuler wrote: > > Wednesday, February 13, 2019 1:43 PM, Alejandro Lucero: > >> Subject: Re: [dpdk-dev] [PATCH 0/6] introduce DMA memory mapping for > >> external memory > >> > >> On Wed, Feb 13, 2019 at 9:11 AM Shahaf Shuler > <shah...@mellanox.com> > >> wrote: > >> > >>> This series is in continue to RFC[1]. > >>> > >>> The DPDK APIs expose 3 different modes to work with memory used for > >> DMA: > >>> > >>> 1. Use the DPDK owned memory (backed by the DPDK provided > >> hugepages). > >>> This memory is allocated by the DPDK libraries, included in the DPDK > >>> memory system (memseg lists) and automatically DMA mapped by the > >> DPDK > >>> layers. > >>> > >>> 2. Use memory allocated by the user and register to the DPDK memory > >>> systems. This is also referred as external memory. Upon registration > >>> of the external memory, the DPDK layers will DMA map it to all > >>> needed devices. > >>> > >>> 3. Use memory allocated by the user and not registered to the DPDK > >>> memory system. This is for users who wants to have tight control on > >>> this memory. The user will need to explicitly call DMA map function > >>> in order to register such memory to the different devices. > >>> > >>> The scope of the patch focus on #3 above. > >>> > >>> > >> Why can not we have case 2 covering case 3? > > > > Because it is not our choice rather the DPDK application. > > We could not allow it, and force the application to register their external > memory to the DPDK memory management system. However IMO it will be > wrong. > > The use case exists - some application wants to manage their memory by > themselves. w/o the extra overhead of rte_malloc, without creating a special > socket to populate the memory and without redundant API calls to > rte_extmem_*. > > > > Simply allocate chunk of memory, DMA map it to device and that’s it. > > Just a small note: while this sounds good on paper, i should point out that at > least *registering* the memory with DPDK is a necessity. You may see > rte_extmem_* calls as redundant (and i agree, to an extent), but we don't > advertise our PMD's capabilities in a way that makes it easy to determine > whether a particular PMD will or will not work without registering external > memory within DPDK (i.e. does it use > rte_virt2memseg() internally, for example). > > So, extmem register calls are a necessary evil in such case, and IMO should > be called out as required for such external memory usage scenario.
If we are going to force all to use the extmem, then there is no need w/ this API. we can have the PMDs to register when the memory is registered. We can just drop the vfio_dma_map APIs and that's it. > > -- > Thanks, > Anatoly
