Re: [PATCH RESEND 1/3] x86/boot: Add bit fields into xloadflags for 5-level kernel checking

[Thomas Gleixner]

On Fri, 25 Jan 2019, Baoquan He wrote:

> Add two bit fields XLF_5LEVEL and XLF_5LEVEL_ENABLED for 5-level kernel.

These are not bit fields. These are simple bits.

> Bit XLF_5LEVEL indicates if 5-level related code is contained
> in this kernel.
> Bit XLF_5LEVEL_ENABLED indicates if CONFIG_X86_5LEVEL=y is set.

I'm confused. 

> -                     .word XLF0 | XLF1 | XLF23 | XLF4
> +#ifdef CONFIG_X86_64
> +#ifdef CONFIG_X86_5LEVEL
> +#define XLF56 (XLF_5LEVEL|XLF_5LEVEL_ENABLED)
> +#else
> +#define XLF56 XLF_5LEVEL
> +#endif
> +#else
> +#define XLF56 0
> +#endif
> +
> +                     .word XLF0 | XLF1 | XLF23 | XLF4 | XLF56

So this actually stores the bits, but looking at the following patch which
fixes the real issue:

> +     if (!(header->xloadflags & XLF_5LEVEL) && pgtable_l5_enabled()) {
> +             pr_err("Can not jump to old 4-level kernel from 5-level 
> kernel.\n");
> +             return ret;
> +     }

So what is XLF_5LEVEL_ENABLED used for and why does it exist at all?

Thanks,

        tglx