On Wed, 2018-08-22 at 08:58 +0200, Christoph Hellwig wrote: > On Thu, Aug 09, 2018 at 09:54:33AM +1000, Benjamin Herrenschmidt wrote: > > On Mon, 2018-07-30 at 18:38 +0200, Christoph Hellwig wrote: > > > We need to take the DMA offset and encryption bit into account when > > > selecting > > > a zone. Add a helper that takes those into account and use it. > > > > That whole "encryption" stuff seems to be completely specific to the > > way x86 does memory encryption, or am I mistaken ? It's not clear to me > > what that does in practice and how it relates to DMA mappings. > > Not even all of x86, but AMD in particular, Intel does it yet another > way. But it still is easier to take this into the core with a few > overrides than duplicating all the code. > > > I'm also not sure about that whole business with ZONE_DMA and > > ARCH_ZONE_DMA_BITS... > > ZONE_DMA usually (but not always) maps to 24-bits of address space, > if it doesn't (I mostly through about s390 with it's odd 31-bits) > the architecture can override it if it cares). > > > On ppc64, unless you enable swiotlb (which we only do currently on > > some embedded platforms), you have all of memory in ZONE_DMA. > > > > [ 0.000000] Zone ranges: > > [ 0.000000] DMA [mem 0x0000000000000000-0x0000001fffffffff] > > [ 0.000000] DMA32 empty > > [ 0.000000] Normal empty > > [ 0.000000] Device empty > > This is really weird. Why would you wire up ZONE_DMA like this?
We always did :-) It predates my involvement and I think it predates even Pauls. It's quite silly actually since the first powerpc machines actually had ISA devices in them, but that's how it's been for ever. I suppose we could change it but that would mean digging out some old stuff to test. > The general scheme that architectures should implement is: > > ZONE_DMA: Any memory below a magic threshold that is lower than > 32-bit. Only enabled if actually required (usually > either 24-bit for ISA, or some other weird architecture > specific value like 32-bit for S/390) It should have been ZONE_ISA_DMA :-) > ZONE_DMA32: Memory <= 32-bit if the architecture supports more than > 32-bits worth of physical address space. Should generally > be enabled on all 64-bit architectures unless you have > a very good reason not to. Yeah so we sort-of enable the config option but only populate the zone on platforms using swiotlb (freescale stuff). It's a bit messy at the moment I must admit. > ZONE_NORMAL: Everything above 32-bit not falling into HIGHMEM or > MOVEABLE. Cheers, Ben.
