On Fri, Apr 17, 2020 at 12:56 PM Fu, Patrick <patrick...@intel.com> wrote: > > Background > ==================================== > DPDK vhost library implements a user-space VirtIO net backend allowing host > applications to directly communicate with VirtIO front-end in VMs and > containers. However, every vhost enqueue/dequeue operation requires to copy > packet buffers between guest and host memory. The overhead of copying large > bulk of data makes the vhost backend become the I/O bottleneck. DMA engines, > including un-core DMA accelerator, like Crystal Beach DMA (CBDMA) and Data > Streaming Accelerator (DSA), and discrete card general purpose DMA, are > extremely efficient in data movement within system memory. Therefore, we > propose a set of asynchronous DMA data movement API in vhost library for DMA > acceleration. With offloading packet copies in vhost data-path from the CPU > to the DMA engine, which can not only accelerate data transfers, but also > save precious CPU core resources. > > New API Overview > ==================================== > The proposed APIs in the vhost library support various DMA engines to > accelerate data transfers in the data-path. For the higher performance, DMA > engines work in an asynchronous manner, where DMA data transfers and CPU > computations are executed in parallel. The proposed API consists of control > path API and data path API. The control path API includes Registration API > and DMA operation callback, and the data path API includes asynchronous API. > To remove the dependency of vendor specific DMA engines, the DMA operation > callback provides generic DMA data transfer abstractions. To support > asynchronous DMA data movement, the new async API provides asynchronous ring > operation semantic in data-path. To enable/disable DMA acceleration for > virtqueues, users need to use registration API is to register/unregister DMA > callback implementations to the vhost library and bind DMA channels to > virtqueues. The DMA channels used by virtqueues are provided by DPDK > applications, which is backed by virtual or physical DMA devices. > The proposed APIs are consisted of 3 sub-sets: > 1. DMA Registration APIs > 2. DMA Operation Callbacks > 3. Async Data APIs > > DMA Registration APIs > ==================================== > DMA acceleration is per queue basis. DPDK applications need to explicitly > decide whether a virtqueue needs DMA acceleration and which DMA channel to > use. In addition, a DMA channel is dedicated to a virtqueue and a DMA channel > cannot be bound to multiple virtqueues at the same time. To enable DMA > acceleration for a virtqueue, DPDK applications need to implement DMA > operation callbacks for a specific DMA type (e.g. CBDMA) first, then register > the callbacks to the vhost library and bind a DMA channel to a virtqueue, and > finally use the new async API to perform data-path operations on the > virtqueue. > The definitions of registration API are shown below: > int rte_vhost_async_channel_register(int vid, uint16_t queue_id, > struct rte_vdma_device_ops *ops); > > int rte_vhost_async_channel_unregister(int vid, uint16_t queue_id);
We already have multiple DMA implementation over raw dev. Why not make a new dmadev class for DMA acceleration and use it by virtio and any other clients?
