powerpc/hv-24x7: Add sysfs files inside hv-24x7 device to show cpumask

Patch here adds a cpumask attr to hv_24x7 pmu along with ABI documentation.

Primary use to expose the cpumask is for the perf tool which has the
capability to parse the driver sysfs folder and understand the
cpumask file. Having cpumask file will reduce the number of perf command
line parameters (will avoid "-C" option in the perf tool
command line). It can also notify the user which is
the current cpu used to retrieve the counter data.

command:# cat /sys/devices/hv_24x7/interface/cpumask
0
Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Reviewed-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200709051836.723765-3-kjain@linux.ibm.com

Documentation/ABI/testing/sysfs-bus-event_source-devices-hv_24x7

@@ -43,6 +43,13 @@ Description:	read only
 		This sysfs interface exposes the number of cores per chip
 		present in the system.
 
+What:		/sys/devices/hv_24x7/interface/cpumask
+Date:		July 2020
+Contact:	Linux on PowerPC Developer List <linuxppc-dev@lists.ozlabs.org>
+Description:	read only
+		This sysfs file exposes the cpumask which is designated to make
+		HCALLs to retrieve hv-24x7 pmu event counter data.
+
 What:		/sys/bus/event_source/devices/hv_24x7/event_descs/<event-name>
 Date:		February 2014
 Contact:	Linux on PowerPC Developer List <linuxppc-dev@lists.ozlabs.org>

arch/powerpc/perf/hv-24x7.c

@@ -448,6 +448,12 @@ static ssize_t device_show_string(struct device *dev,
 	return sprintf(buf, "%s\n", (char *)d->var);
 }
 
+static ssize_t cpumask_show(struct device *dev,
+			    struct device_attribute *attr, char *buf)
+{
+	return cpumap_print_to_pagebuf(true, buf, &hv_24x7_cpumask);
+}
+
 static ssize_t sockets_show(struct device *dev,
 			    struct device_attribute *attr, char *buf)
 {
@@ -1115,6 +1121,7 @@ static DEVICE_ATTR_RO(domains);
 static DEVICE_ATTR_RO(sockets);
 static DEVICE_ATTR_RO(chipspersocket);
 static DEVICE_ATTR_RO(coresperchip);
+static DEVICE_ATTR_RO(cpumask);
 
 static struct bin_attribute *if_bin_attrs[] = {
 	&bin_attr_catalog,
@@ -1128,6 +1135,7 @@ static struct attribute *if_attrs[] = {
 	&dev_attr_sockets.attr,
 	&dev_attr_chipspersocket.attr,
 	&dev_attr_coresperchip.attr,
+	&dev_attr_cpumask.attr,
 	NULL,
 };