On 7/17/23 7:00 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). > > While 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 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 forcing IOMMU_DOMAIN_DMA to be used for 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> This makes sense to me. Reviewed-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
