On Fri, Aug 30, 2024 at 08:03:25AM GMT, Dave Hansen wrote: > On 8/29/24 01:42, Lorenzo Stoakes wrote: > >> These applications work on x86 because x86 does an implicit 47-bit > >> restriction of mmap() address that contain a hint address that is less > >> than 48 bits. > > You mean x86 _has_ to limit to physically available bits in a canonical > > format 🙂 this will not be the case for 5-page table levels though... > > By "physically available bits" are you referring to the bits that can be > used as a part of the virtual address? "Physically" may not have been > the best choice of words. ;) > > There's a canonical hole in 4-level paging and 5-level paging on x86. > The 5-level canonical hole is just smaller. Yeah sorry this is what I meant! > > Also, I should probably say that the >47-bit mmap() access hint was more > of a crutch than something that we wanted to make ABI forever. We knew > that high addresses might break some apps and we hoped that the list of > things it would break would go down over time so that we could > eventually just let mmap() access the whole address space by default. > > That optimism may have been misplaced. Interesting, thanks. This speaks again I think to it being unwise to rely on these things. I do think the only workable form of this series is a fixed personality-based mapping limit.
