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    SubjectRe: [PATCH 2/2] virtio/virtio_ring: Fix the dma_max_mapping_size call
    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@redhat.com>
    > > Suggested-by: Christoph Hellwig <hch@lst.de>
    >
    > 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.

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