Re: [PATCH 2/2] virtio/virtio_ring: Fix the dma_max_mapping_size call

[Christoph Hellwig]

On Mon, Jul 22, 2019 at 11:33:35AM -0400, Michael S. Tsirkin wrote:
> On Mon, Jul 22, 2019 at 04:55:09PM +0200, Eric Auger wrote:
> > Do not call dma_max_mapping_size for devices that have no DMA
> > mask set, otherwise we can hit a NULL pointer dereference.
> > 
> > This occurs when a virtio-blk-pci device is protected with
> > a virtual IOMMU.
> > 
> > Fixes: e6d6dd6c875e ("virtio: Introduce virtio_max_dma_size()")
> > Signed-off-by: Eric Auger <eric.auger@xxxxxxxxxx>
> > Suggested-by: Christoph Hellwig <hch@xxxxxx>
> 
> Christoph, I wonder why did you suggest this?
> The connection between dma_mask and dma_max_mapping_size
> is far from obvious.  The documentation doesn't exist.
> Do we really have to teach all users about this hack?
> Why not just make dma_max_mapping_size DTRT?

Because we should not call into dma API functions for devices that
are not DMA capable.