[tip: x86/mm] x86/realmode: Add comment for Global bit usage in trampoline_pgd

["tip-bot2 for Joerg Roedel" <tip-bot2@linutronix.de>]

The following commit has been merged into the x86/mm branch of tip:

Commit-ID:     9de4999050b5f2e847c84372c6a1aa1fe32bb269
Gitweb:        https://git.kernel.org/tip/9de4999050b5f2e847c84372c6a1aa1fe32bb269
Author:        Joerg Roedel <jroedel@suse.de>
AuthorDate:    Thu, 02 Dec 2021 16:32:23 +01:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Sat, 04 Dec 2021 13:50:08 +01:00

x86/realmode: Add comment for Global bit usage in trampoline_pgd

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@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211202153226.22946-2-joro@8bytes.org
---
 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 1895986..4ba024d 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