On Mon, Feb 14, 2022 at 11:00:59AM -0400, Jason Gunthorpe wrote: > On Mon, Feb 14, 2022 at 03:23:07PM +0100, Joerg Roedel wrote: > > > Device drivers calling into iommu_attach_device() is seldom a good > > idea. In this case the sound device has some generic hardware > > interface so that an existing sound driver can be re-used. Making this > > driver call iommu-specific functions for some devices is something hard > > to justify. > > Er, so this is transparent to the generic sound device? I guess > something fixed up the dma_api on that device to keep working? Right, this is completly transparent to the sound device. The IOMMU code will not set dma_ops on the device because it uses a direct mapping and so the standard implementation will be used. > But, then, the requirement is that nobody is using the dma API when we > make this change? That is the tricky part. DMA-API keeps working after the change is made, because the new domain is also direct mapped. The new domain just has the ability to assign host page-tables to device PASIDs, so that DMA requests with a PASID TLP will be remapped. It was actually a requirement for this code that when it jumps in, the DMA-API mappings stay live. And the reason a direct mapping is used at all is that the page-table walker of the IOMMU is a two-dimensional walker, which will treat the addresses found in the host page-tables as IO-virtual an translates them through the underlying page-table. So to use host-pagetables the underlying mapping must be direct mapped. > I don't think it matters how big/small the group is, only that when we > change the domain we know everything flowing through the domain is > still happy. Yes, that matters. The group size matters too for DMA-API performance. If two devices compete for the same lock in the allocator and/or the same cached magazines, things will slow down. That only matters for high-throughput devices, but still... Regards, Joerg
