Re: [PATCH 11/24] alpha: select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE

[Magnus Lindholm]

Hi Matt,

On Tue, Sep 1, 2026 at 5:51 PM Matt Turner <mattst88@xxxxxxxxx> wrote:
>
> Alpha has a clean split between user and kernel address spaces: user
> space occupies 0x0000000000000000-0x000003ffffffffff (TASK_SIZE), while
> kernel space uses KSEG and vmalloc regions near the top of the 64-bit
> address space. These are strictly non-overlapping.
>
> This enables optimizations in BPF and tracing that can skip address
> range checks when user and kernel pointers are distinguishable by
> address alone.
>

I tested this on an AlphaServer ES40. The kernel built and booted
successfully with CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE=y.

I verified that userspace mappings were below TASK_SIZE
(0x0000040000000000), while the kernel text was in the high Alpha
kernel address range:

  0x000002000025a000  highest observed userspace mapping
  0xfffffc0000310000  _stext

This confirms that the observed user and kernel virtual address ranges
are non-overlapping.

Tested-by: Magnus Lindholm <linmag7@xxxxxxxxx>
Reviewed-by: Magnus Lindholm <linmag7@xxxxxxxxx>