On Thu, May 13, 2021 at 01:22:51PM -0700, Jacob Pan wrote: > Hi Tony, > > On Thu, 13 May 2021 12:57:49 -0700, "Luck, Tony" <[email protected]> > wrote: > > > On Thu, May 13, 2021 at 12:46:21PM -0700, Jacob Pan wrote: > > > It seems there are two options: > > > 1. Add a new IOMMU API to set up a system PASID with a *separate* IOMMU > > > page table/domain, mark the device is PASID only with a flag. Use DMA > > > APIs to explicit map/unmap. Based on this PASID-only flag, Vendor IOMMU > > > driver will decide whether to use system PASID domain during map/unmap. > > > Not clear if we also need to make IOVA==kernel VA. > > > > > > 2. Add a new IOMMU API to setup a system PASID which points to > > > init_mm.pgd. This API only allows trusted device to bind with the > > > system PASID at its own risk. There is no need for DMA API. This is the > > > same as the current code except with an explicit API. > > > > > > Which option? > > > > Option #1 looks cleaner to me. Option #2 gives access to bits > > of memory that the users of system PASID shouldn't ever need > > to touch ... just map regions of memory that the kernel has > > a "struct page" for. > > > > What does "use DMA APIs to explicitly map/unmap" mean? Is that > > for the whole region? > > > If we map the entire kernel direct map during system PASID setup, then we > don't need to use DMA API to map/unmap certain range. > > I was thinking this system PASID page table could be on-demand. The mapping > is built by explicit use of DMA map/unmap APIs.
Option 1 should be the PASID works exactly like a normal RID and uses all the normal DMA APIs and IOMMU mechanisms, whatever the platform implements. This might mean an iommu update on every operation or not. > > I'm expecting that once this system PASID has been initialized, > > then any accelerator device with a kernel use case would use the > > same PASID. I.e. DSA for page clearing, IAX for ZSwap compression > > & decompression, etc. > > > OK, sounds like we have to map the entire kernel VA with struct page as you > said. So we still by-pass DMA APIs, can we all agree on that? Option 2 should be the faster option, but not available in all cases. Option 1 isn't optional. DMA and IOMMU code has to be portable and this is the portable API. If you want to do option 1 and option 2 then give it a go, but in most common cases with the IOMMU in a direct map you shouldn't get a notable performance win. Jason _______________________________________________ iommu mailing list [email protected] https://lists.linuxfoundation.org/mailman/listinfo/iommu
