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

[Jason Wang <jasowang@redhat.com>]

On Mon, Jan 10, 2022 at 5:59 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Fri, Jan 07, 2022 at 02:33:00PM +0800, Xuan Zhuo wrote:
> > 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?
>
> > 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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