Re: [PATCH net-next v3 06/27] virtio_ring: introduce virtqueue_get_buf_ctx_dma()

[Jason Wang <jasowang@redhat.com>]

On Mon, Jan 22, 2024 at 2:12 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>
> On Mon, 22 Jan 2024 12:18:51 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > On Tue, Jan 16, 2024 at 3:47 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> > >
> > > On Thu, 11 Jan 2024 16:34:09 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > > > On Fri, Dec 29, 2023 at 3:31 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> > > > >
> > > > > introduce virtqueue_get_buf_ctx_dma() to collect the dma info when
> > > > > get buf from virtio core for premapped mode.
> > > > >
> > > > > If the virtio queue is premapped mode, the virtio-net send buf may
> > > > > have many desc. Every desc dma address need to be unmap. So here we
> > > > > introduce a new helper to collect the dma address of the buffer from
> > > > > the virtio core.
> > > > >
> > > > > Because the BAD_RING is called (that may set vq->broken), so
> > > > > the relative "const" of vq is removed.
> > > > >
> > > > > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > > > > ---
> > > > >  drivers/virtio/virtio_ring.c | 174 +++++++++++++++++++++++++----------
> > > > >  include/linux/virtio.h       |  16 ++++
> > > > >  2 files changed, 142 insertions(+), 48 deletions(-)
> > > > >
> > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > > > index 51d8f3299c10..1374b3fd447c 100644
> > > > > --- a/drivers/virtio/virtio_ring.c
> > > > > +++ b/drivers/virtio/virtio_ring.c
> > > > > @@ -362,6 +362,45 @@ static struct device *vring_dma_dev(const struct vring_virtqueue *vq)
> > > > >         return vq->dma_dev;
> > > > >  }
> > > > >
> > > > > +/*
> > > > > + *     use_dma_api premapped -> do_unmap
> > > > > + *  1. false       false        false
> > > > > + *  2. true        false        true
> > > > > + *  3. true        true         false
> > > > > + *
> > > > > + * Only #3, we should return the DMA info to the driver.
> > > >
> > > > Btw, I guess you meant "#3 is false" here?
> > > >
> > > > And could we reduce the size of these 3 * 3 matrices? It's usually a
> > > > hint that the code is not optmized.
> > >
> > > On the process of doing dma map, we force the (use_dma_api, premapped).
> > >
> > > if premapped:
> > >      virtio core skip dma map
> > > else:
> > >         if use_dma_api:
> > >                 do dma map
> > >         else:
> > >                 work with the physical address.
> > >
> > > Here we force the (premapped, do_unmap).
> > >
> > > do_unmap is an optimization. We just check this to know should we do dma unmap
> > > or not.
> > >
> > > Now, we introduced an new case, when the virtio core skip dma unmap,
> > > we may need to return the dma info to the driver. That just occur when
> > > the (premapped, do_unmap) is (true, false). Because that the (premmaped,
> > > do_unmap) may be (false, false).
> > >
> > > For the matrices, I just want to show where the do_unmap comes from.
> > > That is a optimization, we use this many places, not to check (use_dma_api,
> > > premapped) on the process of doing unmap. And only for the case #3, we should
> > > return the dma info to drivers.
> >
> > Ok, it tries to ease the life of the readers.
> >
> > I wonder if something like
> >
> > bool virtqueue_needs_unmap() can help, it can judge based on the value
> > of use_dma_api and premapped.
>
>
> I think not too much.
>
> Because do_unmap is for this.
>
>
>
> +static bool vring_need_unmap(struct vring_virtqueue *vq,
> +                            struct virtio_dma_head *dma,
> +                            dma_addr_t addr, unsigned int length)
> +{
> +       if (vq->do_unmap)
> +               return true;
>
> Before this, we is to judge whether we should do unmap or not.
> After this, we is to judge whehter we should return dma info to driver or not.
>
> If you want to simplify this function, I will say no.
>
> If you want to replace "do_unmap" with virtqueue_needs_unmap(), I will say ok.

That's my point.

> But I think we donot need to do that.

Just a suggestion, and you can move the comment above there.

Thanks

>
> +
> +       if (!vq->premapped)
> +               return false;
> +
> +       if (!dma)
> +               return false;
> +
> +       if (unlikely(dma->next >= dma->num)) {
> +               BAD_RING(vq, "premapped vq: collect dma overflow: %pad %u\n",
> +                        &addr, length);
> +               return false;
> +       }
> +
> +       dma->items[dma->next].addr = addr;
> +       dma->items[dma->next].length = length;
> +
> +       ++dma->next;
> +
> +       return false;
> +}
>
>
> Thanks.
>
>
> >
> > Thanks
> >
> > >
> > > Thanks.
> > >
> > > >
> > > > Thanks
> > > >
> > > >
> > >
> >
>