On 9/28/23 10:31 AM, Niklas Schnelle wrote: > ISM devices are virtual PCI devices used for cross-LPAR communication. > Unlike real PCI devices ISM devices do not use the hardware IOMMU but > inspects IOMMU translation tables directly on IOTLB flush (s390 RPCIT > instruction). > > ISM devices keep their DMA allocations static and only very rarely DMA > unmap at all. For each IOTLB flush that occurs after unmap the ISM > devices will however inspect the area of the IOVA space indicated by the > flush. This means that for the global IOTLB flushes used by the flush > queue mechanism the entire IOVA space would be inspected. In principle > this would be fine, albeit potentially unnecessarily slow, it turns out > however that ISM devices are sensitive to seeing IOVA addresses that are > currently in use in the IOVA range being flushed. Seeing such in-use > IOVA addresses will cause the ISM device to enter an error state and > become unusable. > > Fix this by claiming IOMMU_CAP_DEFERRED_FLUSH only for non-ISM devices. > This makes sure IOTLB flushes only cover IOVAs that have been unmapped > and also restricts the range of the IOTLB flush potentially reducing > latency spikes. > > Signed-off-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx> Looks like my review tag from v12 got dropped on accident -- no changes since prior version and still looks good to me so Reviewed-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
