powerpc: Add ppc_strict_facility_enable boot option

Add a boot option that strictly manages the MSR unavailable bits.
This catches kernel uses of FP/Altivec/SPE that would otherwise
corrupt user state.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Documentation/kernel-parameters.txt

@@ -2978,6 +2978,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			may be specified.
 			Format: <port>,<port>....
 
+	ppc_strict_facility_enable
+			[PPC] This option catches any kernel floating point,
+			Altivec, VSX and SPE outside of regions specifically
+			allowed (eg kernel_enable_fpu()/kernel_disable_fpu()).
+			There is some performance impact when enabling this.
+
 	print-fatal-signals=
 			[KNL] debug: print fatal signals
 

arch/powerpc/include/asm/reg.h

@@ -1214,6 +1214,15 @@ static inline void mtmsr_isync(unsigned long val)
 				     : "r" ((unsigned long)(v)) \
 				     : "memory")
 
+extern void msr_check_and_set(unsigned long bits);
+extern bool strict_msr_control;
+extern void __msr_check_and_clear(unsigned long bits);
+static inline void msr_check_and_clear(unsigned long bits)
+{
+	if (strict_msr_control)
+		__msr_check_and_clear(bits);
+}
+
 static inline unsigned long mfvtb (void)
 {
 #ifdef CONFIG_PPC_BOOK3S_64

arch/powerpc/include/asm/switch_to.h

@@ -4,6 +4,8 @@
 #ifndef _ASM_POWERPC_SWITCH_TO_H
 #define _ASM_POWERPC_SWITCH_TO_H
 
+#include <asm/reg.h>
+
 struct thread_struct;
 struct task_struct;
 struct pt_regs;
@@ -26,15 +28,15 @@ extern void enable_kernel_spe(void);
 extern void load_up_spe(struct task_struct *);
 extern void switch_booke_debug_regs(struct debug_reg *new_debug);
 
-static inline void disable_kernel_fp(void) { }
-static inline void disable_kernel_altivec(void) { }
-static inline void disable_kernel_spe(void) { }
-static inline void disable_kernel_vsx(void) { }
-
 #ifdef CONFIG_PPC_FPU
 extern void flush_fp_to_thread(struct task_struct *);
 extern void giveup_fpu(struct task_struct *);
 extern void __giveup_fpu(struct task_struct *);
+static inline void disable_kernel_fp(void)
+{
+	msr_check_and_clear(MSR_FP);
+}
+
 #else
 static inline void flush_fp_to_thread(struct task_struct *t) { }
 static inline void giveup_fpu(struct task_struct *t) { }
@@ -45,6 +47,10 @@ static inline void __giveup_fpu(struct task_struct *t) { }
 extern void flush_altivec_to_thread(struct task_struct *);
 extern void giveup_altivec(struct task_struct *);
 extern void __giveup_altivec(struct task_struct *);
+static inline void disable_kernel_altivec(void)
+{
+	msr_check_and_clear(MSR_VEC);
+}
 #else
 static inline void flush_altivec_to_thread(struct task_struct *t) { }
 static inline void giveup_altivec(struct task_struct *t) { }
@@ -53,6 +59,10 @@ static inline void __giveup_altivec(struct task_struct *t) { }
 
 #ifdef CONFIG_VSX
 extern void flush_vsx_to_thread(struct task_struct *);
+static inline void disable_kernel_vsx(void)
+{
+	msr_check_and_clear(MSR_FP|MSR_VEC|MSR_VSX);
+}
 #else
 static inline void flush_vsx_to_thread(struct task_struct *t)
 {
@@ -63,6 +73,10 @@ static inline void flush_vsx_to_thread(struct task_struct *t)
 extern void flush_spe_to_thread(struct task_struct *);
 extern void giveup_spe(struct task_struct *);
 extern void __giveup_spe(struct task_struct *);
+static inline void disable_kernel_spe(void)
+{
+	msr_check_and_clear(MSR_SPE);
+}
 #else
 static inline void flush_spe_to_thread(struct task_struct *t) { }
 static inline void giveup_spe(struct task_struct *t) { }

arch/powerpc/kernel/process.c

@@ -87,7 +87,19 @@ static void check_if_tm_restore_required(struct task_struct *tsk)
 static inline void check_if_tm_restore_required(struct task_struct *tsk) { }
 #endif /* CONFIG_PPC_TRANSACTIONAL_MEM */
 
-static void msr_check_and_set(unsigned long bits)
+bool strict_msr_control;
+EXPORT_SYMBOL(strict_msr_control);
+
+static int __init enable_strict_msr_control(char *str)
+{
+	strict_msr_control = true;
+	pr_info("Enabling strict facility control\n");
+
+	return 0;
+}
+early_param("ppc_strict_facility_enable", enable_strict_msr_control);
+
+void msr_check_and_set(unsigned long bits)
 {
 	unsigned long oldmsr = mfmsr();
 	unsigned long newmsr;
@@ -103,7 +115,7 @@ static void msr_check_and_set(unsigned long bits)
 		mtmsr_isync(newmsr);
 }
 
-static void msr_check_and_clear(unsigned long bits)
+void __msr_check_and_clear(unsigned long bits)
 {
 	unsigned long oldmsr = mfmsr();
 	unsigned long newmsr;
@@ -118,6 +130,7 @@ static void msr_check_and_clear(unsigned long bits)
 	if (oldmsr != newmsr)
 		mtmsr_isync(newmsr);
 }
+EXPORT_SYMBOL(__msr_check_and_clear);
 
 #ifdef CONFIG_PPC_FPU
 void giveup_fpu(struct task_struct *tsk)