[Patch v2] x86/head/64: remove redundant check on level2_kernel_pgt's _PAGE_PRESENT bit

[Patch v2] x86/head/64: remove redundant check on level2_kernel_pgt's _PAGE_PRESENT bit

[Wei Yang]

This patch tries to remove a redundant check on kernel code's PMD
_PAGE_PRESENT attribute before fix up.

Current process looks like this:

    pmd in [0, _text)
        unset _PAGE_PRESENT
    pmd in [_text, _end]
        if (_PAGE_PRESENT)
            fix up delta
    pmd in (_end, 512)
        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

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
CC: Ingo Molnar <mingo@kernel.org>
CC: Steve Wahl <steve.wahl@hpe.com>

---
v2: adjust the change log to emphasize the redundant check
---
 arch/x86/kernel/head64.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index a817ed0724d1..bac33ec19aa2 100644
--- 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;
 
 	/* invalidate pages after the kernel image */
 	for (; i < PTRS_PER_PMD; i++)
-- 
2.34.1

Re: [Patch v2] x86/head/64: remove redundant check on level2_kernel_pgt's _PAGE_PRESENT bit

[Kirill A . Shutemov]

On Thu, May 23, 2024 at 08:37:52AM +0000, Wei Yang wrote:
> This patch tries to remove a redundant check on kernel code's PMD
> _PAGE_PRESENT attribute before fix up.

Tries? s/This patch tries to r/R/

> Current process looks like this:
> 
>     pmd in [0, _text)
>         unset _PAGE_PRESENT
>     pmd in [_text, _end]
>         if (_PAGE_PRESENT)
>             fix up delta
>     pmd in (_end, 512)
>         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

level2_kernel_pgt compiled with _PAGE_PRESENT set. The check is
redundant.


-- 
  Kiryl Shutsemau / Kirill A. Shutemov

Re: [Patch v2] x86/head/64: remove redundant check on level2_kernel_pgt's _PAGE_PRESENT bit

[Wei Yang]

Thanks for comment.

On Thu, May 23, 2024 at 01:33:28PM +0300, Kirill A . Shutemov wrote:
>On Thu, May 23, 2024 at 08:37:52AM +0000, Wei Yang wrote:
>> This patch tries to remove a redundant check on kernel code's PMD
>> _PAGE_PRESENT attribute before fix up.
>
>Tries? s/This patch tries to r/R/

Will use this one.

>
>> Current process looks like this:
>> 
>>     pmd in [0, _text)
>>         unset _PAGE_PRESENT
>>     pmd in [_text, _end]
>>         if (_PAGE_PRESENT)
>>             fix up delta
>>     pmd in (_end, 512)
>>         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
>
>level2_kernel_pgt compiled with _PAGE_PRESENT set. The check is
>redundant.
>

Will use this one, thanks

>
>-- 
>  Kiryl Shutsemau / Kirill A. Shutemov

-- 
Wei Yang
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