Subject: Re: [PATCH] virt: acrn: Fix vCPU removing code build error

From: Shuo A Liu <[email-masked]>
Date: Fri, 12 Feb 2021 18:58:53 +0800

Hi Greg,

On Fri 12.Feb'21 at  8:52:33 +0100, Greg Kroah-Hartman wrote:
>On Fri, Feb 12, 2021 at 12:57:24PM +0800, [email-masked] wrote:
>> From: Shuo Liu <[email-masked]>
>>
>> vCPU removing code depends on CONFIG_HOTPLUG_CPU as it uses remove_cpu()
>> and add_cpu(). Make the vCPU removing interface building with
>> CONFIG_HOTPLUG_CPU.
>>
>> ../drivers/virt/acrn/hsm.c: In function ‘remove_cpu_store’:
>> ../drivers/virt/acrn/hsm.c:389:3: error: implicit declaration of function ‘remove_cpu’; [-Werror=implicit-function-declaration]
>>    remove_cpu(cpu);
>>
>> ../drivers/virt/acrn/hsm.c:402:2: error: implicit declaration of function ‘add_cpu’; [-Werror=implicit-function-declaration]
>>    add_cpu(cpu);
>>
>> Fixes: 279dcf693ac7 ("virt: acrn: Introduce an interface for Service VM to control vCPU")
>> Reported-by: Randy Dunlap <[email-masked]>
>> Acked-by: Randy Dunlap <[email-masked]> # build-tested
>> Signed-off-by: Shuo Liu <[email-masked]>
>> ---
>>  drivers/virt/acrn/hsm.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/drivers/virt/acrn/hsm.c b/drivers/virt/acrn/hsm.c
>> index 1f6b7c54a1a4..e340788aacdf 100644
>> --- a/drivers/virt/acrn/hsm.c
>> +++ b/drivers/virt/acrn/hsm.c
>> @@ -372,6 +372,7 @@ static int acrn_dev_release(struct inode *inode, struct file *filp)
>>  	return 0;
>>  }
>>
>> +#ifdef CONFIG_HOTPLUG_CPU
>>  static ssize_t remove_cpu_store(struct device *dev,
>>  				struct device_attribute *attr,
>>  				const char *buf, size_t count)
>> @@ -403,9 +404,12 @@ static ssize_t remove_cpu_store(struct device *dev,
>>  	return ret;
>>  }
>>  static DEVICE_ATTR_WO(remove_cpu);
>> +#endif
>>
>>  static struct attribute *acrn_attrs[] = {
>> +#ifdef CONFIG_HOTPLUG_CPU
>>  	&dev_attr_remove_cpu.attr,
>> +#endif
>>  	NULL
>>  };
>>
>>
>
>Shouldn't the real solution for this be that remove_cpu() and add_cpu()
>have function prototypes for when this is not enabled in the kernel
>build?

Something like this in linux/cpu.h?

diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index 3aaa0687e8df..94a578a96202 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -108,6 +108,8 @@ static inline void cpu_maps_update_done(void)
 {
 }

+static inline int add_cpu(unsigned int cpu) { return 0;}
+
 #endif /* CONFIG_SMP */
 extern struct bus_type cpu_subsys;

@@ -137,6 +139,7 @@ static inline int  cpus_read_trylock(void) { return
true; }
 static inline void lockdep_assert_cpus_held(void) { }
 static inline void cpu_hotplug_disable(void) { }
 static inline void cpu_hotplug_enable(void) { }
+static inline int remove_cpu(unsigned int cpu) { return -EPERM; }
 static inline void smp_shutdown_nonboot_cpus(unsigned int primary_cpu)
{ }
 #endif /* !CONFIG_HOTPLUG_CPU */

>
>Putting #ifdef in .c files like this is not a good idea at all.
>
>Then, at runtime, you can determine if you need to create this sysfs
>file or not, as you do not want to expose it to userspace if the kernel
>can not handle it, right?

Right. I don't want to expose the sysfs to userspace if the kernel built
w/o CONFIG_HOTPLUG_CPU. But how to implement that if #ifdef is not used?
misc_register() creates sysfs with .groups (acrn_attr_groups)
unconditionally, then userspace can see the interface even it doesn't
work.

Thanks
shuo