IOMMU_QI_ENTRIES
next = (tail + 1) % IOMMU_QI_ENTRIES;
nfree = IOMMU_QI_ENTRIES - (tail - head) - 1;
tail = (tail + 1) % IOMMU_QI_ENTRIES;
#define IOMMU_QI_SIZE (IOMMU_QI_ENTRIES * sizeof(struct qi_entry))