Re: [TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout

Matthew Wilcox
2025-09-10T14:04:59Z

On Wed, Sep 10, 2025 at 03:10:05PM +0200, Linus Walleij wrote:
> This is is done so that we can unmap *most* of the kernel
> memory but *keep* the VMALLOC area, so that e.g. exceptions
> can still execute and save to stack when they occur, and we can
> call into the kernel to execute said exceptions.
>
> This in practice means the area 0xf1000000-0xffffffff
> which also includes the high exception vectors that must for
> natural reasons also be kept in the mapping.
> What we have mapped today (all of the time) is
> 0xc0000000-0xffffffff.
>
> A minimal 256 MB mapping from 0xf0000000-0xffffffff
> should be able to cover this.

Can I ask where the MMIO (PCI?) window lives?