"Arnd Bergmann" [off-list ref] writes:

On Mon, Sep 9, 2024, at 23:22, Charlie Jenkins wrote:

quoted

On Fri, Sep 06, 2024 at 10:52:34AM +0100, Lorenzo Stoakes wrote:

quoted

On Fri, Sep 06, 2024 at 09:14:08AM GMT, Arnd Bergmann wrote:
The intent is to optionally be able to run a process that keeps higher bits
free for tagging and to be sure no memory mapping in the process will
clobber these (correct me if I'm wrong Charlie! :)

...

Let's see what the other architectures do and then come up with
a way that fixes the pointer tagging case first on those that are
broken. We can see if there needs to be an extra flag after that.
Here is what I found:

- x86_64 uses DEFAULT_MAP_WINDOW of BIT(47), uses a 57 bit
  address space when an addr hint is passed.
- arm64 uses DEFAULT_MAP_WINDOW of BIT(47) or BIT(48), returns
  higher 52-bit addresses when either a hint is passed or
  CONFIG_EXPERT and CONFIG_ARM64_FORCE_52BIT is set (this
  is a debugging option)
- ppc64 uses a DEFAULT_MAP_WINDOW of BIT(47) or BIT(48),
  returns 52 bit address when an addr hint is passed

   
It's 46 or 47 depending on PAGE_SIZE (4K or 64K):

  $ git grep "define DEFAULT_MAP_WINDOW_USER64" arch/powerpc/include/asm/task_size_64.h
  arch/powerpc/include/asm/task_size_64.h:#define DEFAULT_MAP_WINDOW_USER64        TASK_SIZE_128TB
  arch/powerpc/include/asm/task_size_64.h:#define DEFAULT_MAP_WINDOW_USER64        TASK_SIZE_64TB

cheers