On Wed, 2019-04-10 at 08:43 +0200, h...@lst.de wrote: > On Tue, Apr 09, 2019 at 05:24:48PM +0000, Thomas Hellstrom wrote: > > > Note that this only affects external, untrusted devices. But > > > that > > > may include eGPU, > > > > What about discrete graphics cards, like Radeon and Nvidia? Who > > gets to > > determine what's trusted? > > Based on firmware tables. discrete graphics would not qualify unless > they are attached through thunderbolt bridges or external PCIe ports. > > > GPU libraries traditionally have been taking care of the CPU > > mapping > > caching modes since the first AGP drivers. GPU MMU ptes commonly > > support various caching options and pages are changing caching mode > > dynamically. > > So even if the DMA layer needs to do the remapping, couldn't we do > > that > > on-demand when needed with a simple interface? > > The problem is that there is no "simple" interface as the details > depend on the architecture. We have the following base variants > to create coherent memory: > > 1) do nothing - this works on x86-like platforms where I/O is > always > coherent > 2) use a special kernel segment, after flushing the caches for the > normal segment, done on platforms like mips that have this > special segment > 3) remap the existing kernel direct mapping, after flushing the > caches, done by openrisc and in some cases arm32 > 4) create a new mapping in vmap or ioremap space after flushing the > caches - done by most architectures with an MMU and non-coherent > devices > 5) use a special pool of uncached memory set aside by the hardware > or firmware - done by most architectures without an MMU but with > non-coherent devices >
Understood. Unfortunately IMO this severly limits the use of the dma_alloc_coherent() method. > So that is just five major variants with a lot of details on how > it is done in practice. Add to that that many of the operations > are fairly expensive and need to be pre-loaded. > > > > That being said: your driver already uses the dma coherent API > > > under various circumstances, so you already have the above > > > issues. > > > > Yes, but they are hidden behind driver options. We can't have > > someone > > upgrade their kernel and suddenly things don't work anymore, That > > said, > > I think the SWIOTLB case is rare enough for the below solution to > > be > > acceptable, although the TTM check for the coherent page pool being > > available still needs to remain. > > So can you please respin a version acceptable to you and submit it > for 5.1 ASAP? Otherwise I'll need to move ahead with the simple > revert. I will. I need to do some testing to investigate how to best choose between the options, but will have something ready for 5.1. /Thomas _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
