linux-kernel - [PATCH 16/17] sysfs: Provide write method for paravirt

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Message-ID: <20251119124449.1149616-17-sshegde@linux.ibm.com>
Date: Wed, 19 Nov 2025 18:14:48 +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: [PATCH 16/17] sysfs: Provide write method for paravirt

This is a debug 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 allows checking some corner cases by providing custom
cpumasks which isn't possible with arch method.

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>
---
This is currently not meant be merged, since paravirt sysfs file is meant
to be Read-Only. Hence the documentation hasn't changed. If this method is
really helpful, then can consider including it depending on the
discussion.

drivers/base/cpu.c | 47 +++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 46 insertions(+), 1 deletion(-)

diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index c216e13c4e2d..766584c85051 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 paravirt_store(struct device *dev,
+			      struct device_attribute *attr,
+			      const char *buf, size_t count)
+{
+	cpumask_var_t temp_mask;
+	int retval = 0;
+	int cpu;
+
+	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 */
+		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 paravirt_show(struct device *dev,
 				   struct device_attribute *attr, char *buf)
 {
 	return sysfs_emit(buf, "%*pbl\n", cpumask_pr_args(cpu_paravirt_mask));
 }
-static DEVICE_ATTR_RO(paravirt);
+static DEVICE_ATTR_RW(paravirt);
 #endif
 
 const struct bus_type cpu_subsys = {
-- 
2.47.3