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

[Greg KH]

On Wed, Nov 19, 2025 at 01:38:24PM +0530, Shrikanth Hegde wrote:
> Hi Greg.
> 
> On 11/19/25 1:12 PM, Greg KH wrote:
> > On Wed, Nov 19, 2025 at 11:50:59AM +0530, Shrikanth Hegde wrote:
> > > 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 at linux.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);
> > 
> > DEVICE_ATTR_RW()?
> 
> ok.
> 
> > 
> > And where is the documentation update for this sysfs file change?
> > 
> 
> [RFC PATCH v4 11/17] has the documentation of this sysfs file.
> https://lore.kernel.org/all/20251119062100.1112520-12-sshegde@linux.ibm.com/

So a rfc patch has the documentation for a change that you don't want to
have applied?  This is an odd series, how are we supposed to review
this?

> This is a helper patch. This helps to verify functionality of any combination
> of CPUs being marked as paravirt which helped me to test some corner cases.

I don't think I have ever seen a "helper patch" to know what to do with
it :(

thanks,

greg k-h