linux-kernel - [PATCH 03/40] x86/mm/pat: Document the (currently) EFI-only code path

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Date:	Mon, 25 Apr 2016 21:06:35 +0100
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H . Peter Anvin" <hpa@...or.com>
Cc:	Matt Fleming <matt@...eblueprint.co.uk>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>
Subject: [PATCH 03/40] x86/mm/pat: Document the (currently) EFI-only code path

It's not at all obvious that populate_pgd() and friends are only
executed when mapping EFI virtual memory regions or that no other
pageattr callers pass a ->pgd value.

Reported-by: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Matt Fleming <matt@...eblueprint.co.uk>
---
 arch/x86/mm/pageattr.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index 01be9ec3bf79..a1f0e1d0ddc2 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -1125,8 +1125,14 @@ static int populate_pgd(struct cpa_data *cpa, unsigned long addr)
 static int __cpa_process_fault(struct cpa_data *cpa, unsigned long vaddr,
 			       int primary)
 {
-	if (cpa->pgd)
+	if (cpa->pgd) {
+		/*
+		 * Right now, we only execute this code path when mapping
+		 * the EFI virtual memory map regions, no other users
+		 * provide a ->pgd value. This may change in the future.
+		 */
 		return populate_pgd(cpa, vaddr);
+	}
 
 	/*
 	 * Ignore all non primary paths.
-- 
2.7.3