Re: Re: [PATCH] virtio_ring: use u32 for virtio_max_dma_size

[zhenwei pi]

On 5/10/23 11:26, 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.

The max DMA size is a attribute of a virtio device rather than any VQ, 
so I guess virtio_max_dma_size(const struct virtio_device *vdev) is clear.

On the other hand, if changing the parameter to vq, we need select a VQ, 
then the question is:
1, which VQ to select? VQ0 or a random one? this leads confusing.
2, The virtio spec defines: Each device can have zero or more virtqueues


> >   {
> > -	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

--
zhenwei pi