On 2019-02-21 12:34 p.m., Thomas Hellstrom wrote: > On Thu, 2019-02-21 at 16:57 +0000, Kuehling, Felix wrote: >> On 2019-02-21 2:59 a.m., Koenig, Christian wrote: >>> On x86 with HIGHMEM there is no dma32 zone. Why do we need one on >>>>> x86_64? Can we make x86_64 more like HIGHMEM instead? >>>>> >>>>> Regards, >>>>> Felix >>>>> >>>> IIRC with x86, the kernel zone is always smaller than any dma32 >>>> zone, >>>> so we'd always exhaust the kernel zone before dma32 anyway. >>>> >>>> Not sure why we have dma32 on x86 without highmem, though. sounds >>>> superflous but harmless. >>> Well DMA32 denotes memory which is accessible by devices who can >>> only do >>> 32bit addressing. And IIRC we can actually do DMA32 to highmem >>> since >>> something like 2.4.*. >>> >>> Because of this it is actually irrelevant if you have highmem or >>> not, >>> what matters for DMA32 is if you have an IOMMU or not. >> Are you saying we should have a dma32_zone even on x86 with HIGHMEM? >> >> >>> So even on x86_64 you actually do need the DMA32 zone if you don't >>> have >>> an IOMMU which remaps all memory for devices which can't directly >>> address it. >> Why is DMA32 special in this way? For example AMD GFX8 GPUs support >> 40-bit DMA. But we don't have a special zone for that. > If you're running on a non-IOMMU system with physical memory addresses >> 40 bits, and tell the DMA subsystem that you need to restrict to 40 > bits, it will probably start using bounce buffers for streaming DMA > (which won't work with most graphics drivers), or for > dma_alloc_coherent(), it will probably use memory from the DMA32 zone. OK, then why is it not needed when CONFIG_HIGHMEM is defined? I found that there is a CONFIG_ZONE_DMA32 parameter. Maybe we should use that to decide whether to account for the DMA32 zone in TTM. It is set on x86_64 and a number of other 64-bit architectures. >> How common is it to have devices that need DMA32 on an x86_64 system? > IIRC All devices using dma_alloc_coherent() allocate DMA32 memory > unless they explicitly set the dma coherent mask to something larger. > Like Christian says, if an IOMMU is present and enabled, the need for > the DMA32 zone goes away. In theory at least. Thanks. I read up a bit on DMA32 and memory zones in general. I found that there is a lowmem_reserve_ratio feature that protects against normal page allocations overflowing into lowmem zones. There is some documentation in Documentation/scsctl/vm.txt (search for lowmem_reserve_ratio). The protected amount of memory in each zone can be seen in /proc/zoneinfo. With that, can we conclude that we don't need to count ttm_mem_global_alloc against the dma32 zone. Thanks, Felix > > /Thomas > > >> Regards, >> Felix >> >> >>> Regards, >>> Christian. >>> >>>> /Thomas >>>> >>>> _______________________________________________ amd-gfx mailing list amd-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/amd-gfx
