[RFC PATCH v4 11/17] sysfs: Add paravirt CPU file

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Add paravirt file in /sys/devices/system/cpu.

This offers
- User can quickly check which CPUs are marked as paravirt.
- Userspace algorithm such as sched_ext or with isolcpus could
  use the mask and make decision.
- daemon such as irqbalance could use this mask and don't spread
  irq's into paravirt CPUs.

For example:
cat /sys/devices/system/cpu/paravirt
600-719      <<< arch marked these are paravirt.

cat /sys/devices/system/cpu/paravirt
             <<< No paravirt CPUs at the moment.

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
 Documentation/ABI/testing/sysfs-devices-system-cpu |  9 +++++++++
 drivers/base/cpu.c                                 | 12 ++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index 8aed6d94c4cd..1da77430b776 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -777,3 +777,12 @@ Date:		Nov 2022
 Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
 Description:
 		(RO) the list of CPUs that can be brought online.
+
+What:		/sys/devices/system/cpu/paravirt
+Date:		Sep 2025
+Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
+Description:
+		(RO) the list of CPUs that are current marked as paravirt CPUs.
+		These CPUs are not meant to be used at the moment due to
+		contention of underlying physical CPU resource. Dynamically
+		changes to reflect the current situation.
diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index fa0a2eef93ac..59ceae217b22 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -374,6 +374,15 @@ static int cpu_uevent(const struct device *dev, struct kobj_uevent_env *env)
 }
 #endif
 
+#ifdef CONFIG_PARAVIRT
+static ssize_t print_paravirt_cpus(struct device *dev,
+				   struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%*pbl\n", cpumask_pr_args(cpu_paravirt_mask));
+}
+static DEVICE_ATTR(paravirt, 0444, print_paravirt_cpus, NULL);
+#endif
+
 const struct bus_type cpu_subsys = {
 	.name = "cpu",
 	.dev_name = "cpu",
@@ -513,6 +522,9 @@ static struct attribute *cpu_root_attrs[] = {
 #endif
 #ifdef CONFIG_GENERIC_CPU_AUTOPROBE
 	&dev_attr_modalias.attr,
+#endif
+#ifdef CONFIG_PARAVIRT
+	&dev_attr_paravirt.attr,
 #endif
 	NULL
 };
-- 
2.47.3