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

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Date: Thu, 23 May 2024 07:43:59 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Wei Yang <richard.weiyang@...il.com>, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, x86@...nel.org,
	linux-kernel@...r.kernel.org, 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 Wed, May 22, 2024 at 10:33:20PM +0200, Thomas Gleixner wrote:
>On Wed, May 22 2024 at 14:06, Wei Yang wrote:
>> On Wed, May 22, 2024 at 11:58:01AM +0200, Thomas Gleixner wrote:
>>>
>>>What's the actual problem you are trying to solve?
>>
>> Not a problem. It tries to remove some duplicate check.
>
>I assume you mean redundant check, right?

Yep, you are right.

>
>The changelog should explain that. I really could not figure out
>what this is about.
>
>>>>  	/* 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.
>>>
>>
>> Agree. While we are sure then range here must present.
>>
>> The whole process looks like this
>>
>>     pmd in [0, _text)
>>         unset _PAGE_PRESENT
>>     pmd in [_text, _end]
>>         fix up delta
>>     pmd in (_end, 256)
>>         unset _PAGE_PRESENT
>>
>> Since we have compiled in _PAGE_PRESENT in this page table, it is not
>> necessary to check _PAGE_PRESENT again before fixing up delta.
>
>That wants to be in the change log.
>

Sure, I would put this in the change log in next version.

>Referencing the history of the code is definitely interesting and you
>did a great job on decoding it, but for the change itself the only
>relevant information is that all PMDs between _text and _end are marked
>present because the whole table is marked so.
>
>> BTW, if one entry between _text and _end is not present, we will failed to
>> fixing the kernel code pmd entry, which will lead to some problem.
>
>It does not because a non-present entry does not care about the load
>delta obviously.
>
>Thanks,
>
>        tglx

-- 
Wei Yang
Help you, Help me