On Mon, Aug 04, 2025 at 04:37:56AM +0100, Matthew Wilcox wrote: > On Sun, Aug 03, 2025 at 12:59:06PM -0300, Jason Gunthorpe wrote: > > Matthew, do you think it makes sense to introduce types to make this > > clearer? We have two kinds of values that a phys_addr_t can store - > > something compatible with kmap_XX_phys(), and something that isn't. > > I was with you up until this point. And then you said "What if we have > a raccoon that isn't a raccoon" and my brain derailed.
I though it was clear.. kmap_local_pfn(phys >> PAGE_SHIFT) phys_to_virt(phys) Does not work for all values of phys. It definately illegal for non-cachable MMIO. Agree? There is a subset of phys that is cachable and has struct page that is usable with kmap_local_pfn()/etc phys is always this: > - CPU untranslated. This is the "physical" address. Physical address > 0 is what the CPU sees when it drives zeroes on the memory bus. But that is a pure HW perspective. It doesn't say which of our SW APIs are allowed to use this address. We have callchains in DMA API land that want to do a kmap at the bottom. It would be nice to mark the whole call chain that the phys_addr being passed around is actually required to be kmappable. Because if you pass a non-kmappable MMIO backed phys it will explode in some way on some platforms. > > We clearly have these two different ideas floating around in code, > > page tables, etc. > No. No, we don't. I've never heard of this asininity before. Welcome to the fun world of cachable and non-cachable memory. Consider, today we can create struct pages of type MEMORY_DEVICE_PCI_P2PDMA for non-cachable MMIO. I think today you "can" use kmap to establish a cachable mapping in the vmap. But it is *illegal* to establish a cachable CPU mapping of MMIO. Archs are free to MCE if you do this - speculative cache line load of MMIO can just error in HW inside the interconnect. So, the phys_addr is always a "CPU untranslated physical address" but the cachable/non-cachable cases, or DRAM vs MMIO, are sometimes semantically very different things for the SW! Jason
