[PATCH v7 09/18] x86/entry/64: Add the READ_MSR_GSBASE macro

[Chang S. Bae]

Factor out the RDMSR-based GSBASE read into a new macro.

Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
---
 arch/x86/entry/calling.h  |  9 +++++++++
 arch/x86/entry/entry_64.S | 12 ++++++++----
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index efb0d1b..d119729 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -337,6 +337,15 @@ For 32-bit we have the following conventions - kernel is built with
 #endif
 .endm
 
+.macro READ_MSR_GSBASE save_reg:req
+	movl	$MSR_GS_BASE, %ecx
+	/* Read MSR specified by %ecx into %edx:%eax */
+	rdmsr
+	.ifnc \save_reg, %edx
+	movl	%edx, \save_reg
+	.endif
+.endm
+
 #endif /* CONFIG_X86_64 */
 
 .macro STACKLEAK_ERASE
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 20e45d9..568a491 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1159,11 +1159,15 @@ ENTRY(paranoid_entry)
 	cld
 	PUSH_AND_CLEAR_REGS save_ret=1
 	ENCODE_FRAME_POINTER 8
+
 	movl	$1, %ebx
-	movl	$MSR_GS_BASE, %ecx
-	rdmsr
-	testl	%edx, %edx
-	js	1f				/* negative -> in kernel */
+	/*
+	 * The kernel-enforced convention is a negative GSBASE indicates
+	 * a kernel value.
+	 */
+	READ_MSR_GSBASE save_reg=%edx
+	testl	%edx, %edx	/* Negative -> in kernel */
+	js	1f
 	SWAPGS
 	xorl	%ebx, %ebx
 
-- 
2.7.4