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Date:   Wed, 10 May 2023 00:06:54 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
        zhenwei pi <pizhenwei@bytedance.com>,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, Joerg Roedel <jroedel@suse.de>
Subject: Re: [PATCH] virtio_ring: use u32 for virtio_max_dma_size
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On Wed, May 10, 2023 at 12:04:50PM +0800, Jason Wang wrote:
> On Wed, May 10, 2023 at 11:44 AM Michael S. Tsirkin <mst@redhat.com> 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@bytedance.com> 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@suse.de>
> > > > Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
> > > > ---
> > > >  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

Just to make things clear I'm merging fixes for this
release but cleanups belong in the next one.

> >
> > > >  {
> > > > -   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
> > > >
> >