Re: [PATCH 0/9] Kconfig symbol clean-up on ./arch/x86/

[Dave Hansen <dave.hansen@intel.com>]

On 8/3/21 4:35 AM, Lukas Bulwahn wrote:
>   - a reference to STRICT_IOMEM in arch/x86/mm/init.c
>     unclear to me: which exact config this refers to

Are you referring to the reference in this comment?

> +       /*
> +        * This must follow RAM test, since System RAM is considered a
> +        * restricted resource under CONFIG_STRICT_IOMEM.
> +        */
> +       if (iomem_is_exclusive(pagenr << PAGE_SHIFT)) {
> +               /* Low 1MB bypasses iomem restrictions. */
> +               if (pagenr < 256)
> +                       return 1;
...

That came from here:

> commit a4866aa812518ed1a37d8ea0c881dc946409de94
> Author: Kees Cook <keescook@chromium.org>
> Date:   Wed Apr 5 09:39:08 2017 -0700
>
>     mm: Tighten x86 /dev/mem with zeroing reads

Which also added this hunk:

>  #ifdef CONFIG_STRICT_DEVMEM
> +static inline int page_is_allowed(unsigned long pfn)
> +{
> +       return devmem_is_allowed(pfn);
> +}

and talks about CONFIG_STRICT_DEVMEM in the changelog:

>     mm: Tighten x86 /dev/mem with zeroing reads
>     
>     Under CONFIG_STRICT_DEVMEM, reading System RAM through /dev/mem is
>     disallowed. However, on x86, the first 1MB was always allowed for BIOS
...

It's a pretty safe guess that STRICT_IOMEM refers to CONFIG_STRICT_DEVMEM.