powerpc/pseries/svm: Export guest SVM status to user space via sysfs

User space might want to know it's running in a secure VM.  It can't do
a mfmsr because mfmsr is a privileged instruction.

The solution here is to create a cpu attribute:

/sys/devices/system/cpu/svm

which will read 0 or 1 based on the S bit of the current CPU.
Signed-off-by: Ryan Grimm <grimm@linux.vnet.ibm.com>
Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190820021326.6884-12-bauerman@linux.ibm.com

Documentation/ABI/testing/sysfs-devices-system-cpu

@@ -562,3 +562,13 @@ Description:	Umwait control
 			  or C0.2 state. The time is an unsigned 32-bit number.
 			  Note that a value of zero means there is no limit.
 			  Low order two bits must be zero.
+
+What:		/sys/devices/system/cpu/svm
+Date:		August 2019
+Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
+		Linux for PowerPC mailing list <linuxppc-dev@ozlabs.org>
+Description:	Secure Virtual Machine
+
+		If 1, it means the system is using the Protected Execution
+		Facility in POWER9 and newer processors. i.e., it is a Secure
+		Virtual Machine.

arch/powerpc/kernel/sysfs.c

@@ -19,6 +19,7 @@
 #include <asm/smp.h>
 #include <asm/pmc.h>
 #include <asm/firmware.h>
+#include <asm/svm.h>
 
 #include "cacheinfo.h"
 #include "setup.h"
@@ -715,6 +716,23 @@ static struct device_attribute pa6t_attrs[] = {
 #endif /* HAS_PPC_PMC_PA6T */
 #endif /* HAS_PPC_PMC_CLASSIC */
 
+#ifdef CONFIG_PPC_SVM
+static ssize_t show_svm(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%u\n", is_secure_guest());
+}
+static DEVICE_ATTR(svm, 0444, show_svm, NULL);
+
+static void create_svm_file(void)
+{
+	device_create_file(cpu_subsys.dev_root, &dev_attr_svm);
+}
+#else
+static void create_svm_file(void)
+{
+}
+#endif /* CONFIG_PPC_SVM */
+
 static int register_cpu_online(unsigned int cpu)
 {
 	struct cpu *c = &per_cpu(cpu_devices, cpu);
@@ -1058,6 +1076,8 @@ static int __init topology_init(void)
 	sysfs_create_dscr_default();
 #endif /* CONFIG_PPC64 */
 
+	create_svm_file();
+
 	return 0;
 }
 subsys_initcall(topology_init);