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