Re: [PATCH] x86/head/64: level2_kernel_pgt's kernel area is built with _PAGE_PRESENT set

Wei Yang
2024-05-22T14:06:17Z

On Wed, May 22, 2024 at 11:58:01AM +0200, Thomas Gleixner wrote:
>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?

Not a problem. It tries to remove some duplicate check.

>
>> /* 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.

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.

>Thanks,
>
> tglx