linux-kernel - [HELPER PATCH 1] sysfs: Provide write method for paravirt

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Message-ID: <20251119062100.1112520-17-sshegde@linux.ibm.com>
Date: Wed, 19 Nov 2025 11:50:59 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Cc: sshegde@...ux.ibm.com, mingo@...hat.com, peterz@...radead.org,
        juri.lelli@...hat.com, vincent.guittot@...aro.org, tglx@...utronix.de,
        yury.norov@...il.com, maddy@...ux.ibm.com, srikar@...ux.ibm.com,
        gregkh@...uxfoundation.org, pbonzini@...hat.com, seanjc@...gle.com,
        kprateek.nayak@....com, vschneid@...hat.com, iii@...ux.ibm.com,
        huschle@...ux.ibm.com, rostedt@...dmis.org, dietmar.eggemann@....com,
        christophe.leroy@...roup.eu
Subject: [HELPER PATCH 1] sysfs: Provide write method for paravirt

This is helper patch which could be used to set the range of CPUs as
paravirt. One could make use of this for quick testing of this infra
instead of writing arch specific code.

This is currently not meant be merged, since paravirt sysfs file is meant
to be Read-Only.

echo 100-200,600-700 >  /sys/devices/system/cpu/paravirt
cat /sys/devices/system/cpu/paravirt
100-200,600-700

echo > /sys/devices/system/cpu/paravirt
cat /sys/devices/system/cpu/paravirt

Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
---
 drivers/base/cpu.c | 48 ++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 46 insertions(+), 2 deletions(-)

diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index 59ceae217b22..043e4f4ce1a9 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -375,12 +375,57 @@ static int cpu_uevent(const struct device *dev, struct kobj_uevent_env *env)
 #endif
 
 #ifdef CONFIG_PARAVIRT
+static ssize_t store_paravirt_cpus(struct device *dev,
+				   struct device_attribute *attr,
+				   const char *buf, size_t count)
+{
+	cpumask_var_t temp_mask;
+	int retval = 0;
+
+	if (!alloc_cpumask_var(&temp_mask, GFP_KERNEL))
+		return -ENOMEM;
+
+	retval = cpulist_parse(buf, temp_mask);
+	if (retval)
+		goto free_mask;
+
+	/* ALL cpus can't be marked as paravirt */
+	if (cpumask_equal(temp_mask, cpu_online_mask)) {
+		retval = -EINVAL;
+		goto free_mask;
+	}
+	if (cpumask_weight(temp_mask) > num_online_cpus()) {
+		retval = -EINVAL;
+		goto free_mask;
+	}
+
+	/* No more paravirt cpus */
+	if (cpumask_empty(temp_mask)) {
+		cpumask_copy((struct cpumask *)&__cpu_paravirt_mask, temp_mask);
+	} else {
+		cpumask_copy((struct cpumask *)&__cpu_paravirt_mask, temp_mask);
+
+		/* Enable tick on nohz_full cpu */
+		int cpu;
+		for_each_cpu(cpu, temp_mask) {
+			if (tick_nohz_full_cpu(cpu))
+				tick_nohz_dep_set_cpu(cpu, TICK_DEP_BIT_SCHED);
+		}
+	}
+
+	retval = count;
+
+free_mask:
+	free_cpumask_var(temp_mask);
+	return retval;
+}
+
 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);
+static DEVICE_ATTR(paravirt, 0644, print_paravirt_cpus, store_paravirt_cpus);
 #endif
 
 const struct bus_type cpu_subsys = {
@@ -675,7 +720,6 @@ static void __init cpu_register_vulnerabilities(void)
 		put_device(dev);
 	}
 }
-
 #else
 static inline void cpu_register_vulnerabilities(void) { }
 #endif
-- 
2.47.3