Re: [PATCH vhost 02/17] virtio_ring: packed: remove double check of the unmap ops

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On Wed, 31 Jan 2024 17:12:19 +0800, Jason Wang <jasowang@xxxxxxxxxx> wrote:
> On Tue, Jan 30, 2024 at 7:42 PM 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 | 76 ++++++++++++++++++------------------
> >  1 file changed, 39 insertions(+), 37 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 4677831e6c26..7280a1706cca 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -1220,6 +1220,7 @@ 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)
> >  {
> > @@ -1227,33 +1228,18 @@ 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),
> > @@ -1329,7 +1315,7 @@ 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;
> > @@ -1344,10 +1330,11 @@ 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;
>
> Is this a bug fix? Or if we only need to check _F_INDIRECT, we can
> simply avoid doing this by checking vq->indirect && state->indir_desc?
>
> > +
> >         /*
> >          * A driver MUST NOT make the first descriptor in the list
> >          * available before all subsequent descriptors comprising
> > @@ -1388,6 +1375,8 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> >  unmap_release:
> >         err_idx = i;
> >
> > +       WARN_ON(!vring_need_unmap_buffer(vq));
>
> Nitpick, using BUG_ON might be better as it may lead to unexpected
> results which we can't recover from.

the checkpatch.pl does not like BUG_ON.
I have not preference.


>
> > +
> >         for (i = 0; i < err_idx; i++)
> >                 vring_unmap_desc_packed(vq, &desc[i]);
> >
> > @@ -1484,9 +1473,10 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> >                         if (unlikely(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;
> >
> > @@ -1536,6 +1526,8 @@ 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;
> > @@ -1605,7 +1597,9 @@ 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. */
> > @@ -1615,22 +1609,32 @@ static void detach_buf_packed(struct vring_virtqueue *vq,
> >         vq->free_head = id;
> >         vq->vq.num_free += state->num;
> >
> > -       if (unlikely(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 (!(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;
> > +                       }
>
> So before the change, we had:
>
>         if (unlikely(vq->do_unmap)) {
>                 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;
>                 }
>         }
>
> This looks like a bug as we should unmap the indirect descriptor
> regradless of whether do_unmap is true or false.
>
> If yes, we need a independent fix instead of squashing it in this patch?


YES. I noticed this.
I will post a fix to the stable branch.

Thanks.


>
> >                 }
> > -       }
> >
> > -       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);
> > +               }
> > +
>
> Thanks
>





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