linux-kernel - Re: [PATCH] x86/head/64: level2_kernel_pgt's kernel area is built with _PAGE

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Date: Wed, 22 May 2024 11:58:01 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Wei Yang <richard.weiyang@...il.com>, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, Wei Yang
 <richard.weiyang@...il.com>, Vivek Goyal <vgoyal@...ibm.com>, "Kirill A .
 Shutemov" <kirill.shutemov@...ux.intel.com>, Ingo Molnar
 <mingo@...nel.org>, Steve Wahl <steve.wahl@....com>, Borislav Petkov
 <bp@...e.de>
Subject: Re: [PATCH] x86/head/64: level2_kernel_pgt's kernel area is built
 with _PAGE_PRESENT set

On Sat, Mar 23 2024 at 23:26, Wei Yang wrote:
> The code is first introduced in 'commit 1ab60e0f72f7 ("[PATCH] x86-64:
> Relocatable Kernel Support")'.  Then 'commit c88d71508e36b
> ("x86/boot/64: Rewrite startup_64() in C")', convert it to c. And
> 'commit 2aa85f246c181 ("x86/boot/64: Make level2_kernel_pgt pages
> invalid outside kernel area")' limit the range from _text to _end.
>
> Originally, it does the check because the loop iterate the whole
> level2_kernel_pgt, while currently it just fixup the kernel area. This
> area is built with _PAGE_PRESENT set.

What's the actual problem you are trying to solve?

>  	/* fixup pages that are part of the kernel image */
>  	for (; i <= pmd_index((unsigned long)_end); i++)
> -		if (pmd[i] & _PAGE_PRESENT)
> -			pmd[i] += load_delta;
> +		pmd[i] += load_delta;

Fixing up non-present PMDs is a pointless exercise.

Thanks,

        tglx