linux-kernel - [Patch v2] x86/head/64: remove redundant check on level2_kernel_pgt's _PAGE

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Message-Id: <20240523083752.11426-1-richard.weiyang@gmail.com>
Date: Thu, 23 May 2024 08:37:52 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: tglx@...utronix.de,
	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>,
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steve Wahl <steve.wahl@....com>
Subject: [Patch v2] x86/head/64: remove redundant check on level2_kernel_pgt's _PAGE_PRESENT bit

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@...il.com>
CC: Thomas Gleixner <tglx@...utronix.de>
CC: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
CC: Ingo Molnar <mingo@...nel.org>
CC: Steve Wahl <steve.wahl@....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