linux-kernel - [PATCH v4 1/4] x86/realmode: Add comment for Global bit usage in trampline

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Date:   Thu,  2 Dec 2021 16:32:23 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     x86@...nel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        hpa@...or.com, Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Mike Rapoport <rppt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Brijesh Singh <brijesh.singh@....com>,
        linux-kernel@...r.kernel.org, Joerg Roedel <jroedel@...e.de>,
        Joerg Roedel <joro@...tes.org>
Subject: [PATCH v4 1/4] x86/realmode: Add comment for Global bit usage in trampline_pgd

From: Joerg Roedel <jroedel@...e.de>

Document the fact that using the trampoline_pgd will result in the
creation of global TLB entries in the user range of the address
space.

Signed-off-by: Joerg Roedel <jroedel@...e.de>
---
 arch/x86/mm/init.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index 1895986842b9..4ba024d5b63a 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -714,6 +714,11 @@ static void __init memory_map_bottom_up(unsigned long map_start,
 static void __init init_trampoline(void)
 {
 #ifdef CONFIG_X86_64
+	/*
+	 * The code below will alias kernel page-tables in the user-range of the
+	 * address space, including the Global bit. So global TLB entries will
+	 * be created when using the trampoline page-table.
+	 */
 	if (!kaslr_memory_enabled())
 		trampoline_pgd_entry = init_top_pgt[pgd_index(__PAGE_OFFSET)];
 	else
-- 
2.34.0