On x86_64 and arm without those features enabled, the top bits of the pointer must be sign extended. This means that x86_64 by default gives you the top two bytes to play with as long as you don't use the values 0xffff or 0x0000. Attempting to access a pointer whose top 16 bits aren't a sign extension of bit 48 will fault. You can still safely play this game as long as you fix up the pointer before dereferencing it.
"Attempting to access a pointer whose top 16 bits aren't a sign extension of bit 48 will fault."
Currently. Coming soon to an Intel chip near you is 57-bit virtual addressing and 5-level page tables [1]. It would be quite a bug that would only crash on new Intel hardware on probably quite full memory maps where your pointer fix up code wouldn't restore bits 48-57 correctly.
x86 large pages are 2 MB (64-bit) or 4 MB (32-bit). Even larger page variety is 1 GB. They do save a lot of TLB misses, but can be painful to reliably allocate.
> Currently. Coming soon to an Intel chip near you is 57-bit virtual addressing and 5-level page tables [1]. It would be quite a bug that would only crash on new Intel hardware on probably quite full memory maps where your pointer fix up code wouldn't restore bits 48-57 correctly.
Yeah, I'd hope that code that uses such tricks would have an error check for pointers that use the bits it wants to use. Better a clean crash than corruption.
It'd be neat if a program could tell the OS what address range is acceptable. Linux has mmap(..., MAP_32BIT, ...) but obviously that's pretty limited. Maybe something like map(addr, ..., MAP_MAXADDR, ...) which would tell it addr represents the maximum acceptable address to return. So if you intend to use the top 16 bits, you could tell it this mapping can't use those.
Edit: oh, actually, I see they do something kind of like this. https://lwn.net/Articles/717293/ "An application that needs [virtual address beyond 48-bits], and which does not play games with virtual addresses, can provide an address hint above the boundary in a call to mmap(), at which point the kernel will understand that mappings in the upper range are accessible." Still not quite as flexible as I was imagining but not bad.]
On x86_64 and arm without those features enabled, the top bits of the pointer must be sign extended. This means that x86_64 by default gives you the top two bytes to play with as long as you don't use the values 0xffff or 0x0000. Attempting to access a pointer whose top 16 bits aren't a sign extension of bit 48 will fault. You can still safely play this game as long as you fix up the pointer before dereferencing it.