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

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Tue, 11 Jan 2022 10:54:45 +0800, Jason Wang <jasowang@redhat.com> wrote:
> 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.
>

I understand that there is a headroom for receiving packages, which can be used
to put virtio headers. But there is no headroom defined in the direction of
sending packages. I hope Magnus Karlsson can help confirm whether there is any
misunderstanding.

It would be best if we could not use indirect.

Thanks.

> > 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
> >
>
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization