DMAR_PQT_REG
tail = readq(iommu->reg + DMAR_PQT_REG) & PRQ_RING_MASK;
tail = readq(iommu->reg + DMAR_PQT_REG) & PRQ_RING_MASK;
writeq(0ULL, iommu->reg + DMAR_PQT_REG);
writeq(0ULL, iommu->reg + DMAR_PQT_REG);
tail = readq(iommu->reg + DMAR_PQT_REG) & PRQ_RING_MASK;