[PATCH v1] x86/boot: Use pgd_index(__START_KERNEL_map) instead of hardcoded 511

[shengminghu512 <shengminghu512@qq.com>]

From: Shengming Hu <shengminghu512@qq.com>
Date: Wed, 8 Oct 2025 21:46:03 +0800
Subject: [PATCH v1] x86/boot: Use pgd_index(__START_KERNEL_map) instead of
 hardcoded 511

The index 511 is used to set up the kernel high mapping in the
top-level page table. 

Replacing the magic number with pgd_index(__START_KERNEL_map) 
improves code readability and future-proofs the code against 
possible changes in paging structure.

Signed-off-by: Shengming Hu <shengminghu512@qq.com>
---
 arch/x86/kernel/head64.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index fd28b53db..65d4c485a 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -286,7 +286,9 @@ asmlinkage __visible void __init __noreturn x86_64_start_kernel(char * real_mode
 	load_ucode_bsp();
 
 	/* set init_top_pgt kernel high mapping*/
-	init_top_pgt[511] = early_top_pgt[511];
+	unsigned int kernel_pgd = pgd_index(__START_KERNEL_map);
+
+	init_top_pgt[kernel_pgd] = early_top_pgt[kernel_pgd];
 
 	x86_64_start_reservations(real_mode_data);
 }
-- 
2.34.1