Re: [PATCH vhost v4 02/10] virtio_ring: packed: remove double check of the unmap ops

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On Thu, Mar 21, 2024 at 4:21 PM Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, 21 Mar 2024 13:57:06 +0800, Jason Wang <jasowang@xxxxxxxxxx> wrote:
> > On Tue, Mar 12, 2024 at 11:36 AM Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> wrote:
> > >
> > > In the functions vring_unmap_extra_packed and vring_unmap_desc_packed,
> > > multiple checks are made whether unmap is performed and whether it is
> > > INDIRECT.
> > >
> > > These two functions are usually called in a loop, and we should put the
> > > check outside the loop.
> > >
> > > And we unmap the descs with VRING_DESC_F_INDIRECT on the same path with
> > > other descs, that make the thing more complex. If we distinguish the
> > > descs with VRING_DESC_F_INDIRECT before unmap, thing will be clearer.
> > >
> > > 1. only one desc of the desc table is used, we do not need the loop
> > > 2. the called unmap api is difference from the other desc
> > > 3. the vq->premapped is not needed to check
> > > 4. the vq->indirect is not needed to check
> > > 5. the state->indir_desc must not be null
> > >
> > > Signed-off-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> > > ---
> > >  drivers/virtio/virtio_ring.c | 78 ++++++++++++++++++------------------
> > >  1 file changed, 40 insertions(+), 38 deletions(-)
> > >
> > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > index c2779e34aac7..0dfbd17e5a87 100644
> > > --- a/drivers/virtio/virtio_ring.c
> > > +++ b/drivers/virtio/virtio_ring.c
> > > @@ -1214,6 +1214,7 @@ static u16 packed_last_used(u16 last_used_idx)
> > >         return last_used_idx & ~(-(1 << VRING_PACKED_EVENT_F_WRAP_CTR));
> > >  }
> > >
> > > +/* caller must check vring_need_unmap_buffer() */
> > >  static void vring_unmap_extra_packed(const struct vring_virtqueue *vq,
> > >                                      const struct vring_desc_extra *extra)
> > >  {
> > > @@ -1221,33 +1222,18 @@ static void vring_unmap_extra_packed(const struct vring_virtqueue *vq,
> > >
> > >         flags = extra->flags;
> > >
> > > -       if (flags & VRING_DESC_F_INDIRECT) {
> > > -               if (!vq->use_dma_api)
> > > -                       return;
> > > -
> > > -               dma_unmap_single(vring_dma_dev(vq),
> > > -                                extra->addr, extra->len,
> > > -                                (flags & VRING_DESC_F_WRITE) ?
> > > -                                DMA_FROM_DEVICE : DMA_TO_DEVICE);
> > > -       } else {
> > > -               if (!vring_need_unmap_buffer(vq))
> > > -                       return;
> > > -
> > > -               dma_unmap_page(vring_dma_dev(vq),
> > > -                              extra->addr, extra->len,
> > > -                              (flags & VRING_DESC_F_WRITE) ?
> > > -                              DMA_FROM_DEVICE : DMA_TO_DEVICE);
> > > -       }
> > > +       dma_unmap_page(vring_dma_dev(vq),
> > > +                      extra->addr, extra->len,
> > > +                      (flags & VRING_DESC_F_WRITE) ?
> > > +                      DMA_FROM_DEVICE : DMA_TO_DEVICE);
> > >  }
> > >
> > > +/* caller must check vring_need_unmap_buffer() */
> > >  static void vring_unmap_desc_packed(const struct vring_virtqueue *vq,
> > >                                     const struct vring_packed_desc *desc)
> > >  {
> > >         u16 flags;
> > >
> > > -       if (!vring_need_unmap_buffer(vq))
> > > -               return;
> > > -
> > >         flags = le16_to_cpu(desc->flags);
> > >
> > >         dma_unmap_page(vring_dma_dev(vq),
> > > @@ -1323,7 +1309,7 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> > >                         total_sg * sizeof(struct vring_packed_desc),
> > >                         DMA_TO_DEVICE);
> > >         if (vring_mapping_error(vq, addr)) {
> > > -               if (vq->premapped)
> > > +               if (!vring_need_unmap_buffer(vq))
> > >                         goto free_desc;
> > >
> > >                 goto unmap_release;
> > > @@ -1338,10 +1324,11 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> > >                 vq->packed.desc_extra[id].addr = addr;
> > >                 vq->packed.desc_extra[id].len = total_sg *
> > >                                 sizeof(struct vring_packed_desc);
> > > -               vq->packed.desc_extra[id].flags = VRING_DESC_F_INDIRECT |
> > > -                                                 vq->packed.avail_used_flags;
> > >         }
> > >
> > > +       vq->packed.desc_extra[id].flags = VRING_DESC_F_INDIRECT |
> > > +               vq->packed.avail_used_flags;
> > > +
> > >         /*
> > >          * A driver MUST NOT make the first descriptor in the list
> > >          * available before all subsequent descriptors comprising
> > > @@ -1382,6 +1369,8 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> > >  unmap_release:
> > >         err_idx = i;
> > >
> > > +       WARN_ON(!vring_need_unmap_buffer(vq));
> > > +
> > >         for (i = 0; i < err_idx; i++)
> > >                 vring_unmap_desc_packed(vq, &desc[i]);
> > >
> > > @@ -1475,12 +1464,13 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> > >                         desc[i].len = cpu_to_le32(sg->length);
> > >                         desc[i].id = cpu_to_le16(id);
> > >
> > > -                       if (unlikely(vq->use_dma_api)) {
> > > +                       if (vring_need_unmap_buffer(vq)) {
> > >                                 vq->packed.desc_extra[curr].addr = addr;
> > >                                 vq->packed.desc_extra[curr].len = sg->length;
> > > -                               vq->packed.desc_extra[curr].flags =
> > > -                                       le16_to_cpu(flags);
> > >                         }
> > > +
> > > +                       vq->packed.desc_extra[curr].flags = le16_to_cpu(flags);
> > > +
> > >                         prev = curr;
> > >                         curr = vq->packed.desc_extra[curr].next;
> > >
> > > @@ -1530,6 +1520,8 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> > >
> > >         vq->packed.avail_used_flags = avail_used_flags;
> > >
> > > +       WARN_ON(!vring_need_unmap_buffer(vq));
> > > +
> > >         for (n = 0; n < total_sg; n++) {
> > >                 if (i == err_idx)
> > >                         break;
> > > @@ -1599,7 +1591,9 @@ static void detach_buf_packed(struct vring_virtqueue *vq,
> > >         struct vring_desc_state_packed *state = NULL;
> > >         struct vring_packed_desc *desc;
> > >         unsigned int i, curr;
> > > +       u16 flags;
> > >
> > > +       flags = vq->packed.desc_extra[id].flags;
> > >         state = &vq->packed.desc_state[id];
> > >
> > >         /* Clear data ptr. */
> > > @@ -1609,22 +1603,32 @@ static void detach_buf_packed(struct vring_virtqueue *vq,
> > >         vq->free_head = id;
> > >         vq->vq.num_free += state->num;
> > >
> > > -       if (unlikely(vq->use_dma_api)) {
> > > -               curr = id;
> > > -               for (i = 0; i < state->num; i++) {
> > > -                       vring_unmap_extra_packed(vq,
> > > -                                                &vq->packed.desc_extra[curr]);
> > > -                       curr = vq->packed.desc_extra[curr].next;
> > > +       if (!(flags & VRING_DESC_F_INDIRECT)) {
> > > +               if (vring_need_unmap_buffer(vq)) {
> > > +                       curr = id;
> > > +                       for (i = 0; i < state->num; i++) {
> > > +                               vring_unmap_extra_packed(vq,
> > > +                                                        &vq->packed.desc_extra[curr]);
> > > +                               curr = vq->packed.desc_extra[curr].next;
> > > +                       }
> > >                 }
> > > -       }
> > >
> > > -       if (vq->indirect) {
> > > +               if (ctx)
> > > +                       *ctx = state->indir_desc;
> > > +       } else {
> > > +               const struct vring_desc_extra *extra;
> > >                 u32 len;
> > >
> > > +               if (vq->use_dma_api) {
> > > +                       extra = &vq->packed.desc_extra[id];
> > > +                       dma_unmap_single(vring_dma_dev(vq),
> > > +                                        extra->addr, extra->len,
> > > +                                        (flags & VRING_DESC_F_WRITE) ?
> > > +                                        DMA_FROM_DEVICE : DMA_TO_DEVICE);
> > > +               }
> >
> > Theoretically, indirect descriptors could be chained. It is supported
> > without this patch but not here.
>
>
> YES. But now, that is not supported by "add", so I think we
> do not need to think about it.
>
> Thanks.

Well, you need to at least document this somewhere (probably in the changlog).

Thanks






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