linux-kernel - Re: [Patch v3] x86/head/64: remove redundant check on level2_kernel_pgt's _PAGE

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Message-ID: <9e9669a6-de37-4140-bdc5-7d660b8427c3@intel.com>
Date: Mon, 3 Jun 2024 11:50:06 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Wei Yang <richard.weiyang@...il.com>, tglx@...utronix.de,
 mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
 "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
 Ingo Molnar <mingo@...nel.org>, Steve Wahl <steve.wahl@....com>
Subject: Re: [Patch v3] x86/head/64: remove redundant check on
 level2_kernel_pgt's _PAGE_PRESENT bit

On 5/23/24 05:35, Wei Yang wrote:
> --- a/arch/x86/kernel/head64.c
> +++ b/arch/x86/kernel/head64.c
> @@ -260,8 +260,7 @@ unsigned long __head __startup_64(unsigned long physaddr,
>  
>  	/* 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;

So, I think this is correct.  But, man, I wish folks would go through
the git history and make it clear that they understand _how_ the code
got the way it is.

I suspect that the original _PAGE_PRESENT check wasn't even necessary if
cleanup_highmap() really did fix things up.  But this commit:

	2aa85f246c18 ("x86/boot/64: Make level2_kernel_pgt pages invalid
		       outside kernel area")

tweaked things to actively clear out PMDs that weren't populated in
Kirill's original loop.  It didn't touch the _PAGE_PRESENT check.  But
it certainly did imply that the PMD doesn't have any holes in it and
there's nothing int he middle that needs _PAGE_PRESENT cleared.

> level2_kernel_pgt compiled with _PAGE_PRESENT set. The check is
> redundant

This isn't super reassuring.  It also depends on nothing having munged
the page tables up to this point.  The code is also a bit cruel in that
it manipulates two different sets of PMDs with the same 'pmd' variable.

Also, is this comment still accurate after '2aa85f246c18'?

>          * Fixup the kernel text+data virtual addresses. Note that
>          * we might write invalid pmds, when the kernel is relocated
>          * cleanup_highmap() fixes this up along with the mappings
>          * beyond _end.