Hi, On Thu, 2019-02-21 at 20:24 +0000, Kuehling, Felix wrote: > 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. Yes, it indeed looks like that. But then I would suggest removing the DMA32 zone entirely. /Thomas > > Thanks, > Felix > > > > /Thomas > > > > > > > Regards, > > > Felix > > > > > > > > > > Regards, > > > > Christian. > > > > > > > > > /Thomas > > > > > > > > > > _______________________________________________ amd-gfx mailing list amd-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/amd-gfx
