CMD_TAIL_REG
tail = iommu_read_4(iommu, CMD_TAIL_REG);
iommu_write_4(iommu, CMD_TAIL_REG, next);
iommu_read_8(iommu, CMD_TAIL_REG),
iommu_write_4(iommu, CMD_TAIL_REG, 0x00);