powerpc/lib/xor_vmx: Ensure no altivec code executes before enable_kernel_altivec()

The xor_vmx.c file is used for the RAID5 xor operations. In these functions
altivec is enabled to run the operation and then disabled.

The code uses enable_kernel_altivec() around the core of the algorithm, however
the whole file is built with -maltivec, so the compiler is within its rights to
generate altivec code anywhere. This has been seen at least once in the wild:

  0:mon> di $xor_altivec_2
  c0000000000b97d0  3c4c01d9	addis   r2,r12,473
  c0000000000b97d4  3842db30	addi    r2,r2,-9424
  c0000000000b97d8  7c0802a6	mflr    r0
  c0000000000b97dc  f8010010	std     r0,16(r1)
  c0000000000b97e0  60000000	nop
  c0000000000b97e4  7c0802a6	mflr    r0
  c0000000000b97e8  faa1ffa8	std     r21,-88(r1)
  ...
  c0000000000b981c  f821ff41	stdu    r1,-192(r1)
  c0000000000b9820  7f8101ce	stvx    v28,r1,r0		<-- POP
  c0000000000b9824  38000030	li      r0,48
  c0000000000b9828  7fa101ce	stvx    v29,r1,r0
  ...
  c0000000000b984c  4bf6a06d	bl      c0000000000238b8 # enable_kernel_altivec

This patch splits the non-altivec code into xor_vmx_glue.c which calls the
altivec functions in xor_vmx.c. By compiling xor_vmx_glue.c without
-maltivec we can guarantee that altivec instruction will not be executed
outside of the enable/disable block.
Signed-off-by: Matt Brown <matthew.brown.dev@gmail.com>
[mpe: Rework change log and include disassembly]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/lib/Makefile

@@ -37,7 +37,7 @@ obj-$(CONFIG_PPC_LIB_RHEAP) += rheap.o
 
 obj-$(CONFIG_FTR_FIXUP_SELFTEST) += feature-fixups-test.o
 
-obj-$(CONFIG_ALTIVEC)	+= xor_vmx.o
+obj-$(CONFIG_ALTIVEC)	+= xor_vmx.o xor_vmx_glue.o
 CFLAGS_xor_vmx.o += -maltivec $(call cc-option,-mabi=altivec)
 
 obj-$(CONFIG_PPC64) += $(obj64-y)

arch/powerpc/lib/xor_vmx.c

@@ -29,10 +29,7 @@
 #define vector __attribute__((vector_size(16)))
 #endif
 
-#include <linux/preempt.h>
-#include <linux/export.h>
-#include <linux/sched.h>
-#include <asm/switch_to.h>
+#include "xor_vmx.h"
 
 typedef vector signed char unative_t;
 
@@ -64,16 +61,13 @@ typedef vector signed char unative_t;
 		V1##_3 = vec_xor(V1##_3, V2##_3);	\
 	} while (0)
 
-void xor_altivec_2(unsigned long bytes, unsigned long *v1_in,
+void __xor_altivec_2(unsigned long bytes, unsigned long *v1_in,
 		     unsigned long *v2_in)
 {
 	DEFINE(v1);
 	DEFINE(v2);
 	unsigned long lines = bytes / (sizeof(unative_t)) / 4;
 
-	preempt_disable();
-	enable_kernel_altivec();
-
 	do {
 		LOAD(v1);
 		LOAD(v2);
@@ -83,13 +77,9 @@ void xor_altivec_2(unsigned long bytes, unsigned long *v1_in,
 		v1 += 4;
 		v2 += 4;
 	} while (--lines > 0);
-
-	disable_kernel_altivec();
-	preempt_enable();
 }
-EXPORT_SYMBOL(xor_altivec_2);
 
-void xor_altivec_3(unsigned long bytes, unsigned long *v1_in,
+void __xor_altivec_3(unsigned long bytes, unsigned long *v1_in,
 		     unsigned long *v2_in, unsigned long *v3_in)
 {
 	DEFINE(v1);
@@ -97,9 +87,6 @@ void xor_altivec_3(unsigned long bytes, unsigned long *v1_in,
 	DEFINE(v3);
 	unsigned long lines = bytes / (sizeof(unative_t)) / 4;
 
-	preempt_disable();
-	enable_kernel_altivec();
-
 	do {
 		LOAD(v1);
 		LOAD(v2);
@@ -112,13 +99,9 @@ void xor_altivec_3(unsigned long bytes, unsigned long *v1_in,
 		v2 += 4;
 		v3 += 4;
 	} while (--lines > 0);
-
-	disable_kernel_altivec();
-	preempt_enable();
 }
-EXPORT_SYMBOL(xor_altivec_3);
 
-void xor_altivec_4(unsigned long bytes, unsigned long *v1_in,
+void __xor_altivec_4(unsigned long bytes, unsigned long *v1_in,
 		     unsigned long *v2_in, unsigned long *v3_in,
 		     unsigned long *v4_in)
 {
@@ -128,9 +111,6 @@ void xor_altivec_4(unsigned long bytes, unsigned long *v1_in,
 	DEFINE(v4);
 	unsigned long lines = bytes / (sizeof(unative_t)) / 4;
 
-	preempt_disable();
-	enable_kernel_altivec();
-
 	do {
 		LOAD(v1);
 		LOAD(v2);
@@ -146,13 +126,9 @@ void xor_altivec_4(unsigned long bytes, unsigned long *v1_in,
 		v3 += 4;
 		v4 += 4;
 	} while (--lines > 0);
-
-	disable_kernel_altivec();
-	preempt_enable();
 }
-EXPORT_SYMBOL(xor_altivec_4);
 
-void xor_altivec_5(unsigned long bytes, unsigned long *v1_in,
+void __xor_altivec_5(unsigned long bytes, unsigned long *v1_in,
 		     unsigned long *v2_in, unsigned long *v3_in,
 		     unsigned long *v4_in, unsigned long *v5_in)
 {
@@ -163,9 +139,6 @@ void xor_altivec_5(unsigned long bytes, unsigned long *v1_in,
 	DEFINE(v5);
 	unsigned long lines = bytes / (sizeof(unative_t)) / 4;
 
-	preempt_disable();
-	enable_kernel_altivec();
-
 	do {
 		LOAD(v1);
 		LOAD(v2);
@@ -184,8 +157,4 @@ void xor_altivec_5(unsigned long bytes, unsigned long *v1_in,
 		v4 += 4;
 		v5 += 4;
 	} while (--lines > 0);
-
-	disable_kernel_altivec();
-	preempt_enable();
 }
-EXPORT_SYMBOL(xor_altivec_5);

arch/powerpc/lib/xor_vmx.h

+/*
+ * Simple interface to link xor_vmx.c and xor_vmx_glue.c
+ *
+ * Separating these file ensures that no altivec instructions are run
+ * outside of the enable/disable altivec block.
+ */
+
+void __xor_altivec_2(unsigned long bytes, unsigned long *v1_in,
+			     unsigned long *v2_in);
+
+void __xor_altivec_3(unsigned long bytes, unsigned long *v1_in,
+			     unsigned long *v2_in, unsigned long *v3_in);
+
+void __xor_altivec_4(unsigned long bytes, unsigned long *v1_in,
+			     unsigned long *v2_in, unsigned long *v3_in,
+			     unsigned long *v4_in);
+
+void __xor_altivec_5(unsigned long bytes, unsigned long *v1_in,
+			     unsigned long *v2_in, unsigned long *v3_in,
+			     unsigned long *v4_in, unsigned long *v5_in);

arch/powerpc/lib/xor_vmx_glue.c

+/*
+ * Altivec XOR operations
+ *
+ * Copyright 2017 IBM Corp.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version
+ * 2 of the License, or (at your option) any later version.
+ */
+
+#include <linux/preempt.h>
+#include <linux/export.h>
+#include <linux/sched.h>
+#include <asm/switch_to.h>
+#include "xor_vmx.h"
+
+void xor_altivec_2(unsigned long bytes, unsigned long *v1_in,
+		   unsigned long *v2_in)
+{
+	preempt_disable();
+	enable_kernel_altivec();
+	__xor_altivec_2(bytes, v1_in, v2_in);
+	disable_kernel_altivec();
+	preempt_enable();
+}
+EXPORT_SYMBOL(xor_altivec_2);
+
+void xor_altivec_3(unsigned long bytes,  unsigned long *v1_in,
+		   unsigned long *v2_in, unsigned long *v3_in)
+{
+	preempt_disable();
+	enable_kernel_altivec();
+	__xor_altivec_3(bytes, v1_in, v2_in, v3_in);
+	disable_kernel_altivec();
+	preempt_enable();
+}
+EXPORT_SYMBOL(xor_altivec_3);
+
+void xor_altivec_4(unsigned long bytes,  unsigned long *v1_in,
+		   unsigned long *v2_in, unsigned long *v3_in,
+		   unsigned long *v4_in)
+{
+	preempt_disable();
+	enable_kernel_altivec();
+	__xor_altivec_4(bytes, v1_in, v2_in, v3_in, v4_in);
+	disable_kernel_altivec();
+	preempt_enable();
+}
+EXPORT_SYMBOL(xor_altivec_4);
+
+void xor_altivec_5(unsigned long bytes,  unsigned long *v1_in,
+		   unsigned long *v2_in, unsigned long *v3_in,
+		   unsigned long *v4_in, unsigned long *v5_in)
+{
+	preempt_disable();
+	enable_kernel_altivec();
+	__xor_altivec_5(bytes, v1_in, v2_in, v3_in, v4_in, v5_in);
+	disable_kernel_altivec();
+	preempt_enable();
+}
+EXPORT_SYMBOL(xor_altivec_5);