Re: [PATCH v1 0/6] virtio: support advance DMA

[Jason Wang <jasowang@redhat.com>]

On Mon, Feb 21, 2022 at 2:20 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>
> On Mon, 21 Feb 2022 13:59:06 +0800, Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> > On Mon, 21 Feb 2022 11:53:33 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > > On Mon, Feb 21, 2022 at 11:46 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> > > >
> > > > On Mon, 21 Feb 2022 11:32:52 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > > > > On Fri, Feb 18, 2022 at 5:00 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> > > > > >
> > > > > > On Thu, 17 Feb 2022 15:19:44 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > > > > > > On Thu, Feb 10, 2022 at 4:51 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> 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().
> > > > > > >
> > > > > > > I'd suggest rename this feature as "unmanaged DMA".
> > > > > >
> > > > > > OK
> > > > > >
> > > > > > >
> > > > > > > >
> > > > > > > > Record this predma information in extra->flags, which can be skipped when
> > > > > > > > executing dma unmap.
> > > > > > >
> > > > > > > Question still, can we use per-virtqueue flag instead of per
> > > > > > > descriptor flag? If my memory is correct, the answer is yes in the
> > > > > > > discussion for the previous version.
> > > > > > >
> > > > > >
> > > > > > Yes.
> > > > > >
> > > > > > per-virtqueue? I guess it should be per-submit.
> > > > > >
> > > > > > This patch set only adds a flag to desc_extra[head].flags, so that we can know
> > > > > > if we need to unmap dma when we detach.
> > > > >
> > > > > I meant if we can manage to make it per virtqueue, there's no need to
> > > > > maintain per buffer flag.
> > > > >
> > > > > So we know something that needs to be mapped by virtio core itself,
> > > > > e.g the indirect page. Other than this, all the rest could be
> > > > > pre-mapped.
> > > > >
> > > > > For vnet header, it could be mapped by virtio-net which could be still
> > > > > treated as pre mapped DMA since it's not the virtio ring code.
> > > > >
> > > > > Anything I miss here?
> > > >
> > > > I guess, your understanding is that after the queue is reset, the queue is used
> > > > by xsk(AF_XDP), then all commits to this vq are premapped amd address.
> > > >
> > > > This is ok for rx.
> > > >
> > > > But for tx, just like XDP TX, although vq is used by xsk, the kernel also passes
> > > > skb to it at the same time. It is shared.
> > >
> > > Right.
> > >
> > > >
> > > > We can guarantee that the sg of the sgs submitted at one time uses the premapped
> > > > dma address or virtual address uniformly. It is not guaranteed that all the sgs
> > > > to the vq are uniform
> > >
> > > Sorry, I don't understand here. We can let virtio-net do the mapping
> > > even for TX, then from the virtio_ring point of view, it's still
> > > pre-mapped?
> > >
> >
> > Yes, we can do this. My previous thought was to keep the skb path unchanged.

We can listen from Michael and others but I think it would be simpler.

And we can even make the pre-mapped per driver. E.g for virtio-net we
just let the virtio-net driver do the DMA mapping. This allows us to
do a lot of optimizations (e.g page flip) as what other networking
drivers did.

> >
> > Then we can make it clear that in the case of xsk, after completing the queue
> > reset, all the addresses submitted to virtio are the addresses of the completed
> > dma, including the skb case, the dma map operation must be completed first.
> >
> > In this case, I feel like we can do without this patch set.
>
> I originally thought that use_dma_api could be reused, but I found that this is
> not the case. The logic of sg_phys() does not meet our ideas. We still have a
> separate flag.

Just to make sure I understand here, for this flag you mean

1) per buffer
2) per virtqueue
or
3) per device?

Thanks

>
> Thanks.
>
> >
> > Thanks.
> >
> > > Thanks
> > >
> > > >
> > > > Thanks.
> > > >
> > > > >
> > > > > Thanks
> > > > >
> > > > >
> > > > > >
> > > > > > Thanks.
> > > > > >
> > > > > > > Thanks
> > > > > > >
> > > > > > > >
> > > > > > > > v1:
> > > > > > > >    1. All sgs requested at one time are required to be unified PREDMA, and several
> > > > > > > >       of them are not supported to be PREDMA
> > > > > > > >    2. virtio_dma_map() is removed from this patch set and will be submitted
> > > > > > > >       together with the next time AF_XDP supports virtio dma
> > > > > > > >    3. Added patch #2 #3 to remove the check for flags when performing unmap
> > > > > > > >       indirect desc
> > > > > > > >
> > > > > > > > Xuan Zhuo (6):
> > > > > > > >   virtio: rename vring_unmap_state_packed() to
> > > > > > > >     vring_unmap_extra_packed()
> > > > > > > >   virtio: remove flags check for unmap split indirect desc
> > > > > > > >   virtio: remove flags check for unmap packed indirect desc
> > > > > > > >   virtio: virtqueue_add() support predma
> > > > > > > >   virtio: split: virtqueue_add_split() support dma address
> > > > > > > >   virtio: packed: virtqueue_add_packed() support dma address
> > > > > > > >
> > > > > > > >  drivers/virtio/virtio_ring.c | 199 ++++++++++++++++++++++-------------
> > > > > > > >  1 file changed, 126 insertions(+), 73 deletions(-)
> > > > > > > >
> > > > > > > > --
> > > > > > > > 2.31.0
> > > > > > > >
> > > > > > >
> > > > > >
> > > > >
> > > >
> > >
> > _______________________________________________
> > Virtualization mailing list
> > Virtualization@lists.linux-foundation.org
> > https://lists.linuxfoundation.org/mailman/listinfo/virtualization
>

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