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

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Message-ID: <fc2617a6-d6af-4347-8e6d-467396ab8a39@linux.ibm.com>
Date: Wed, 19 Nov 2025 15:26:33 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        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, 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: Re: [HELPER PATCH 1] sysfs: Provide write method for paravirt



On 11/19/25 1:53 PM, Greg KH wrote:
> 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@...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);
>>>
>>> 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?

I added the documentation for sysfs file as the file is read only. The last two
patch are debug patches. So i didn't update the documentation saying it can be written
too. I hope this clears the doubts.

> 
>> 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 :(
> 

Sorry for confusion with the name.

All I wanted to say there was it is debug patch one could use.
Would [RFC PATCH 16/17][DEBUG] would have been a better name?