Re: [PATCH 0/6] virtio: support advance DMA

On Mon, Jan 10, 2022 at 5:59 PM Michael S. Tsirkin [off-list ref] wrote:

On Fri, Jan 07, 2022 at 02:33:00PM +0800, Xuan Zhuo wrote:

quoted

virtqueue_add() only supports virtual addresses, dma is completed in
virtqueue_add().

In some scenarios (such as the AF_XDP scenario), DMA is completed in advance, so
it is necessary for us to support passing the DMA address to virtqueue_add().

This patch set stipulates that if sg->dma_address is not NULL, use this
address as the DMA address. And record this information in extra->flags,
which can be skipped when executing dma unmap.

    extra->flags |= VRING_DESC_F_PREDMA;

But the indirect desc does not have a corresponding extra, so the second and
third patches of this patch set are to allocate the corresponding extra while
allocating the indirect desc. Each desc must have a corresponding extra because
it is possible in an sgs some are advance DMA, while others are virtual
addresses. So we must allocate an extra for each indirect desc.

I didn't realize AF_XDP didn't have space to stuff the header into.
Jason, is that expected?

I might be wrong but it looks to me AF_XDP allows to reserve
sufficient headroom via xdp_umem_reg_v1.

It would be best to fix that, performance is
best if header is linear with the data ...

This looks like a must otherwise we may meet trouble in zerocopy receive.

Thanks

Or maybe we can reduce the use of indirect somewhat, at least while
the ring is mostly empty?

quoted

Xuan Zhuo (6):
  virtio: rename vring_unmap_state_packed() to
    vring_unmap_extra_packed()
  virtio: split: alloc indirect desc with extra
  virtio: packed: alloc indirect desc with extra
  virtio: split: virtqueue_add_split() support dma address
  virtio: packed: virtqueue_add_packed() support dma address
  virtio: add api virtio_dma_map() for advance dma

 drivers/virtio/virtio_ring.c | 387 ++++++++++++++++++++---------------
 include/linux/virtio.h       |   9 +
 2 files changed, 232 insertions(+), 164 deletions(-)

--
2.31.0

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