4 ms·

I think there might be some more hardware-specific nuance here. e.g. /proc/cpuinfo says this on a couple of different x86_64 systems that I checked. address

I think there might be some more hardware-specific nuance here. e.g. /proc/cpuinfo says this on a couple of different x86_64 systems that I checked. address sizes : 36 bits physical, 48 bits virtual address sizes : 40 bits physical, 48 bits virtual PS: I don't understand what this means, btw.

db48x 6y ago
Your CPU can handle 39-bit physical memory addresses (up to 512 GB of physical memory), and 48-bit virtual addresses (256 TB). Your operating system maintains a mapping from virtual to physical addresses, usually arranging the map so that every process has a separate memory space. Pointers are all still 64 bits long though.
- barnacled 6y ago
  In practice the actual available usable address space for userland is 64 TiB due to user/kernel split and the kernel maintaining a virtual mapping of the entire physical address space (minus I/O ranges) [0]. However newer incoming 5-level page intel chips [1] will allow up to 57 bits of address space, 128 PiB in theory though in practice 32 PiB of userland memory. See also [0] for discussion on practical limit for 5-page too! [0]:https://github.com/lorenzo-stoakes/linux-mm-notes/blob/master/virt_layout.md#available-address-space https://github.com/lorenzo-stoakes/linux-mm-notes/blob/maste... [1]:https://en.wikipedia.org/wiki/Intel_5-level_paging https://en.wikipedia.org/wiki/Intel_5-level_paging
  - db48x 6y ago
    True, though /proc/cpuinfo only reports the size, which is ultimately what the CPU cares about. Plus the most relevant limit is what your motherboard and wallet supports, which is often far lower.
    - barnacled 6y ago
      Indeed, and as you say, sensibly speaking you are hardly likely to hit those limits in any likely (esp. home) setup. The actual meaningful limit is usually the CPU physical one as home CPUs very often have stringent memory limits (often 32 GiB or so) and of course you rely on the motherboard's limitations also. Having said that I did write a patch to ensure that the system would boot correctly with 256 TiB of RAM [0] so perhaps I am not always a realist... or dream of the day I can own that system ;) [0]:https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=167dcfc08b0b1f964ea95d410aa496fd78adf475 https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-n...
      - db48x 6y ago
        You're not the only one dreaming; I had to use >200GB of swap on my home system last year.
- sigjuice 6y ago
  So are the 16 leftmost bits of a virtual address always 0?
  - barnacled 6y ago
    They have to be same as the maximum addressable bit, i.e. in the case of 48 bit virtual address size the 48th bit. This is actually kind of a cute way of dividing kernel and userland space as you just set the upper bit to 1 for kernel addresses and 0 for userland. EDIT: Specifically talking about x86-64 here. https://github.com/lorenzo-stoakes/linux-mm-notes/blob/master/virt_layout.md#virtual-address-layout https://github.com/lorenzo-stoakes/linux-mm-notes/blob/maste...
  - amscanne 6y ago
    No, it must be sign-extended from the top bit of the valid set. Otherwise the address is non-canonical.
    - xxpor 6y ago
      This is true for x86-64, not true for other architectures such as arm64. Apple uses the high bits to cryptographicly sign the pointer value.
      - everybodyknows 6y ago
        Fascinating. Does this confer some of the benefits of ECC RAM, for pointer data only — without the hardware cost?
        danielheath 6y ago
        Some (but I believe the advantage is that it’s much harder to inject valid code from a buffer overflow).
        saagarjha 6y ago
        PAC is more about CFI than preventing shellcode injection (which is done through codesigning and memory protection, mostly).
throwaway8581 6y ago
It means that it can address 40-bits of address space worth of physical memory, but that virtual memory addresses can use 48 bits. Physical addresses are just your RAM bytes numbered 1 through whatever. Virtual address space is the address space of a process, which includes mapped physical memory, unmapped pages, guard pages, and other virtual memory tricks.
- hansendc 6y ago
  > Physical addresses are just your RAM bytes numbered 1 through whatever. Not really. There are lots of holes in the physical address map. Look at /proc/iomem. Look at all of the gunk in there at addresses lower than the amount of RAM you have. Look at the highest “System RAM” address. It will be higher than the amount of actual physical RAM that you have.

db48x 6y ago

Your CPU can handle 39-bit physical memory addresses (up to 512 GB of physical memory), and 48-bit virtual addresses (256 TB). Your operating system maintains a mapping from virtual to physical addresses, usually arranging the map so that every process has a separate memory space. Pointers are all still 64 bits long though.

barnacled 6y ago
In practice the actual available usable address space for userland is 64 TiB due to user/kernel split and the kernel maintaining a virtual mapping of the entire physical address space (minus I/O ranges) [0]. However newer incoming 5-level page intel chips [1] will allow up to 57 bits of address space, 128 PiB in theory though in practice 32 PiB of userland memory. See also [0] for discussion on practical limit for 5-page too! [0]:https://github.com/lorenzo-stoakes/linux-mm-notes/blob/master/virt_layout.md#available-address-space https://github.com/lorenzo-stoakes/linux-mm-notes/blob/maste... [1]:https://en.wikipedia.org/wiki/Intel_5-level_paging https://en.wikipedia.org/wiki/Intel_5-level_paging
- db48x 6y ago
  True, though /proc/cpuinfo only reports the size, which is ultimately what the CPU cares about. Plus the most relevant limit is what your motherboard and wallet supports, which is often far lower.
  - barnacled 6y ago
    Indeed, and as you say, sensibly speaking you are hardly likely to hit those limits in any likely (esp. home) setup. The actual meaningful limit is usually the CPU physical one as home CPUs very often have stringent memory limits (often 32 GiB or so) and of course you rely on the motherboard's limitations also. Having said that I did write a patch to ensure that the system would boot correctly with 256 TiB of RAM [0] so perhaps I am not always a realist... or dream of the day I can own that system ;) [0]:https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=167dcfc08b0b1f964ea95d410aa496fd78adf475 https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-n...
    - db48x 6y ago
      You're not the only one dreaming; I had to use >200GB of swap on my home system last year.
sigjuice 6y ago
So are the 16 leftmost bits of a virtual address always 0?
- barnacled 6y ago
  They have to be same as the maximum addressable bit, i.e. in the case of 48 bit virtual address size the 48th bit. This is actually kind of a cute way of dividing kernel and userland space as you just set the upper bit to 1 for kernel addresses and 0 for userland. EDIT: Specifically talking about x86-64 here. https://github.com/lorenzo-stoakes/linux-mm-notes/blob/master/virt_layout.md#virtual-address-layout https://github.com/lorenzo-stoakes/linux-mm-notes/blob/maste...
- amscanne 6y ago
  No, it must be sign-extended from the top bit of the valid set. Otherwise the address is non-canonical.
  - xxpor 6y ago
    This is true for x86-64, not true for other architectures such as arm64. Apple uses the high bits to cryptographicly sign the pointer value.
    - everybodyknows 6y ago
      Fascinating. Does this confer some of the benefits of ECC RAM, for pointer data only — without the hardware cost?
      - danielheath 6y ago
        Some (but I believe the advantage is that it’s much harder to inject valid code from a buffer overflow).
        saagarjha 6y ago
        PAC is more about CFI than preventing shellcode injection (which is done through codesigning and memory protection, mostly).

throwaway8581 6y ago

It means that it can address 40-bits of address space worth of physical memory, but that virtual memory addresses can use 48 bits. Physical addresses are just your RAM bytes numbered 1 through whatever. Virtual address space is the address space of a process, which includes mapped physical memory, unmapped pages, guard pages, and other virtual memory tricks.

hansendc 6y ago
> Physical addresses are just your RAM bytes numbered 1 through whatever. Not really. There are lots of holes in the physical address map. Look at /proc/iomem. Look at all of the gunk in there at addresses lower than the amount of RAM you have. Look at the highest “System RAM” address. It will be higher than the amount of actual physical RAM that you have.