[PATCH vhost v8 00/12] virtio core prepares for AF_XDP

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## About DMA APIs

Now, virtio may can not work with DMA APIs when virtio features do not have

1. I tried to let DMA APIs return phy address by virtio-device. But DMA APIs just
   work with the "real" devices.
2. I tried to let xsk support callballs to get phy address from virtio-net
   driver as the dma address. But the maintainers of xsk may want to use dma-buf
   to replace the DMA APIs. I think that may be a larger effort. We will wait
   too long.

So rethinking this, firstly, we can support premapped-dma only for devices with
VIRTIO_F_ACCESS_PLATFORM. In the case of af-xdp, if the users want to use it,
they have to update the device to support VIRTIO_F_RING_RESET, and they can also
enable the device's VIRTIO_F_ACCESS_PLATFORM feature.

Thanks for the help from Christoph.


XDP socket(AF_XDP) is an excellent bypass kernel network framework. The zero
copy feature of xsk (XDP socket) needs to be supported by the driver. The
performance of zero copy is very good.

ENV: Qemu with vhost.

                   vhost cpu | Guest APP CPU |Guest Softirq CPU | PPS
xmit by sockperf:     90%    |   100%        |                  |  318967
xmit by xsk:          100%   |   30%         |   33%            | 1192064
recv by sockperf:     100%   |   68%         |   100%           |  692288
recv by xsk:          100%   |   33%         |   43%            |  771670

Before achieving the function of Virtio-Net, we also have to let virtio core
support these features:

1. virtio core support premapped
2. virtio core support reset per-queue
3. introduce DMA APIs to virtio core

Please review.


 1. vring_sg_address: check by sg_page(sg) not dma_address. Because 0 is a valid dma address
 2. remove unused code from vring_map_one_sg()

 1. virtqueue_dma_dev() return NULL when virtio is without DMA API.

 1. change the size of the flags to u32.

 1. fix for error handler
 2. add flags to record internal dma mapping

 1. rename map_inter to dma_map_internal
 2. fix: Excess function parameter 'vq' description in 'virtqueue_dma_dev'

 1. add map_inter to struct desc state to reocrd whether virtio core do dma map

 1. based on sgs[0]->dma_address to judgment is premapped
 2. based on extra.addr to judgment to do unmap for no-indirect desc
 3. based on indir_desc to judgment to do unmap for indirect desc
 4. rename virtqueue_get_dma_dev to virtqueue_dma_dev

 1. expose dma device. NO introduce the api for dma and sync
 2. split some commit for review.

Xuan Zhuo (12):
  virtio_ring: split: separate dma codes
  virtio_ring: packed: separate dma codes
  virtio_ring: packed-indirect: separate dma codes
  virtio_ring: split: support premapped
  virtio_ring: packed: support premapped
  virtio_ring: packed-indirect: support premapped
  virtio_ring: remove unused code
  virtio_ring: update document for virtqueue_add_*
  virtio_ring: introduce virtqueue_dma_dev()
  virtio_ring: correct the expression of the description of
  virtio_ring: separate the logic of reset/enable from virtqueue_resize
  virtio_ring: introduce virtqueue_reset()

 drivers/virtio/virtio_ring.c | 375 +++++++++++++++++++++++++----------
 include/linux/virtio.h       |   4 +
 2 files changed, 272 insertions(+), 107 deletions(-)


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