My guess is that the last definition is the one being used. If dma_ops
is NULL, then &dma_ops->is_phys should be exactly 0x78 (which is also
the bad address) if I counted right (struct dma_mapping_ops in
asm-x86_64/dma-mapping.h).
So dma_ops is currently NULL inside domU. Maybe nommu_dma_ops should
be used?
Googling for "xen domu no_iommu_init", this shows up: