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

From: Shuo A Liu <[email-masked]>
Date: Fri, 12 Feb 2021 19:19:21 +0800

On Fri 12.Feb'21 at 12:02:18 +0100, Greg Kroah-Hartman wrote:
>On Fri, Feb 12, 2021 at 06:58:53PM +0800, Shuo A Liu wrote:
>> 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.
>
>Use the is_visible() callback for your attribute group.

Thanks for the guide, is_visible() is good. Seems good i can provide the
callback with 
	return IS_ENABLED(CONFIG_HOTPLUG_CPU);

Will update in v2.

Thanks
shuo