linux-kernel - [PATCH] x86-64: no need to fix up physical addresses if they are correct

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Message-Id: <1425130460-29807-1-git-send-email-kuleshovmail@gmail.com>
Date:	Sat, 28 Feb 2015 19:34:20 +0600
From:	Alexander Kuleshov <kuleshovmail@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...e.de>
Cc:	linux-kernel@...r.kernel.org,
	Alexander Kuleshov <kuleshovmail@...il.com>
Subject: [PATCH] x86-64: no need to fix up physical addresses if they are correct

If kernel doesn't use kASLR and runned on the same address that was
compiled to run, %rbp will be zero and no need to fix physical addresses
in the page tables.

Signed-off-by: Alexander Kuleshov <kuleshovmail@...il.com>
---
 arch/x86/kernel/head_64.S | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 6fd514d9..c0127bc 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -86,6 +86,14 @@ startup_64:
 	jnz	bad_address
 
 	/*
+	 * We have no need to fixup the physical addresses in the page tables
+	 * if there is no difference between the address where kernel compiled
+	 * to run and the actual address where kernel running at.
+	 */
+	cmpq $0x0, %rbp
+	je 1f
+
+	/*
 	 * Fixup the physical addresses in the page table
 	 */
 	addq	%rbp, early_level4_pgt + (L4_START_KERNEL*8)(%rip)
@@ -95,6 +103,7 @@ startup_64:
 
 	addq	%rbp, level2_fixmap_pgt + (506*8)(%rip)
 
+1:
 	/*
 	 * Set up the identity mapping for the switchover.  These
 	 * entries should *NOT* have the global bit set!  This also
-- 
2.3.1.167.g7f4ba4b

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