Re: [PATCH] virtio_ring: use u32 for virtio_max_dma_size

[Jason Wang]

On Wed, May 10, 2023 at 11:44 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Wed, May 10, 2023 at 11:26:54AM +0800, Xuan Zhuo wrote:
> > On Wed, 10 May 2023 10:54:37 +0800, zhenwei pi <pizhenwei@xxxxxxxxxxxxx> wrote:
> > > Both split ring and packed ring use 32bits to describe the length of
> > > a descriptor: see struct vring_desc and struct vring_packed_desc.
> > > This means the max segment size supported by virtio is U32_MAX.
> > >
> > > An example of virtio_max_dma_size in virtio_blk.c:
> > >   u32 v, max_size;
> > >
> > >   max_size = virtio_max_dma_size(vdev);  -> implicit convert
> > >   err = virtio_cread_feature(vdev, VIRTIO_BLK_F_SIZE_MAX,
> > >                              struct virtio_blk_config, size_max, &v);
> > >   max_size = min(max_size, v);
> > >
> > > There is a risk during implicit convert here, once virtio_max_dma_size
> > > returns 4G, max_size becomes 0.
> > >
> > > Fixes: e6d6dd6c875e ("virtio: Introduce virtio_max_dma_size()")
> > > Cc: Joerg Roedel <jroedel@xxxxxxx>
> > > Signed-off-by: zhenwei pi <pizhenwei@xxxxxxxxxxxxx>
> > > ---
> > >  drivers/virtio/virtio_ring.c | 12 ++++++++----
> > >  include/linux/virtio.h       |  2 +-
> > >  2 files changed, 9 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > index c5310eaf8b46..55cfecf030a1 100644
> > > --- a/drivers/virtio/virtio_ring.c
> > > +++ b/drivers/virtio/virtio_ring.c
> > > @@ -289,12 +289,16 @@ static bool vring_use_dma_api(const struct virtio_device *vdev)
> > >     return false;
> > >  }
> > >
> > > -size_t virtio_max_dma_size(const struct virtio_device *vdev)
> > > +u32 virtio_max_dma_size(const struct virtio_device *vdev)
> >
> >
> > LGTM
> >
> > But, should we change the parameter to vq, then use the dma_dev?
> >
> > @Jason
> >
> > Thanks.
> >
>
>
> that would be an unrelated rework.

Probably, but I think it's better to be done on top otherwise we may forget.

Thanks

>
> > >  {
> > > -   size_t max_segment_size = SIZE_MAX;
> > > +   u32 max_segment_size = U32_MAX;
> > >
> > > -   if (vring_use_dma_api(vdev))
> > > -           max_segment_size = dma_max_mapping_size(vdev->dev.parent);
> > > +   if (vring_use_dma_api(vdev)) {
> > > +           size_t max_dma_size = dma_max_mapping_size(vdev->dev.parent);
> > > +
> > > +           if (max_dma_size < max_segment_size)
> > > +                   max_segment_size = max_dma_size;
> > > +   }
> > >
> > >     return max_segment_size;
> > >  }
> > > diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> > > index b93238db94e3..1a605f408329 100644
> > > --- a/include/linux/virtio.h
> > > +++ b/include/linux/virtio.h
> > > @@ -147,7 +147,7 @@ int virtio_device_restore(struct virtio_device *dev);
> > >  #endif
> > >  void virtio_reset_device(struct virtio_device *dev);
> > >
> > > -size_t virtio_max_dma_size(const struct virtio_device *vdev);
> > > +u32 virtio_max_dma_size(const struct virtio_device *vdev);
> > >
> > >  #define virtio_device_for_each_vq(vdev, vq) \
> > >     list_for_each_entry(vq, &vdev->vqs, list)
> > > --
> > > 2.20.1
> > >
>