misc: pvpanic: introduce events device attribue

Suggested by Paolo & Greg, add 'events' device attribute that can be
used to limit which capabilities the driver uses.

Finally, the pvpanic guest driver works by the limitation of both
device capability and user setting.
Signed-off-by: Nzhenwei pi <pizhenwei@bytedance.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Link: https://lore.kernel.org/r/20210110115358.79100-3-pizhenwei@bytedance.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-bus-pci-devices-pvpanic

@@ -10,3 +10,15 @@ Description:
 		Detailed bit definition refers to section <Bit Definition>
 		from pvpanic device specification:
 		https://git.qemu.org/?p=qemu.git;a=blob_plain;f=docs/specs/pvpanic.txt
+
+What:		/sys/devices/pci0000:00/*/QEMU0001:00/events
+Date:		Jan 2021
+Contact:	zhenwei pi <pizhenwei@bytedance.com>
+Description:
+		RW attribute. Set/get which features in-use. This attribute
+		is used to enable/disable feature(s) of pvpanic device.
+		Notice that this value should be a subset of capability.
+
+		Format: %x.
+
+		Also refer to pvpanic device specification.

drivers/misc/pvpanic.c

@@ -20,6 +20,7 @@
 
 static void __iomem *base;
 static unsigned int capability = PVPANIC_PANICKED | PVPANIC_CRASH_LOADED;
+static unsigned int events;
 
 static ssize_t capability_show(struct device *dev,
 			       struct device_attribute *attr, char *buf)
@@ -28,8 +29,34 @@ static ssize_t capability_show(struct device *dev,
 }
 static DEVICE_ATTR_RO(capability);
 
+static ssize_t events_show(struct device *dev,  struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%x", events);
+}
+
+static ssize_t events_store(struct device *dev,  struct device_attribute *attr,
+			    const char *buf, size_t count)
+{
+	unsigned int tmp;
+	int err;
+
+	err = kstrtouint(buf, 16, &tmp);
+	if (err)
+		return err;
+
+	if ((tmp & capability) != tmp)
+		return -EINVAL;
+
+	events = tmp;
+
+	return count;
+
+}
+static DEVICE_ATTR_RW(events);
+
 static struct attribute *pvpanic_dev_attrs[] = {
 	&dev_attr_capability.attr,
+	&dev_attr_events.attr,
 	NULL
 };
 ATTRIBUTE_GROUPS(pvpanic_dev);
@@ -41,7 +68,7 @@ MODULE_LICENSE("GPL");
 static void
 pvpanic_send_event(unsigned int event)
 {
-	if (event & capability)
+	if (event & capability & events)
 		iowrite8(event, base);
 }
 
@@ -90,6 +117,7 @@ static int pvpanic_mmio_probe(struct platform_device *pdev)
 
 	/* initlize capability by RDPT */
 	capability &= ioread8(base);
+	events = capability;
 
 	if (capability)
 		atomic_notifier_chain_register(&panic_notifier_list,