linux-kernel - Re: [PATCH 3/5] [RESEND] x86: qspinlock-paravirt: fix mising-prototype warnings

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Message-Id: <21188533-a56b-484b-8cb7-053f38a4155c@app.fastmail.com>
Date:   Tue, 01 Aug 2023 22:26:36 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Borislav Petkov" <bp@...en8.de>, "Arnd Bergmann" <arnd@...nel.org>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 3/5] [RESEND] x86: qspinlock-paravirt: fix mising-prototype
 warnings

On Tue, Aug 1, 2023, at 21:22, Borislav Petkov wrote:
> On Tue, Jul 25, 2023 at 03:48:35PM +0200, Arnd Bergmann wrote:
>> diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
>> index 89842bb7ec9cc..64a6bba70d183 100644
>> --- a/arch/x86/kernel/paravirt.c
>> +++ b/arch/x86/kernel/paravirt.c
>> @@ -73,11 +73,13 @@ DEFINE_PARAVIRT_ASM(pv_native_read_cr2, "mov %cr2, %rax", .noinstr.text);
>>  
>>  DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key);
>>  
>> +#ifdef CONFIG_SMP
>>  void __init native_pv_lock_init(void)
>>  {
>>  	if (!boot_cpu_has(X86_FEATURE_HYPERVISOR))
>>  		static_branch_disable(&virt_spin_lock_key);
>>  }
>> +#endif
>
> Can you add an empty UP stub instead?
>
> We all have a great aversion against ifdeffery...

There is already a stub for !CONFIG_PARAVIRT in asm/qspinlock.h,
but the problem is that this header does not get included
anywhere in UP configurations. 

The variant below would avoid adding more #ifdefs, by moving
the declaration into asm/paravirt.h to ensure that it's
declared even if there is no caller.

Does this look better to you?

     Arnd

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index b49778664d2be..fc3a377bb9b79 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -739,6 +739,7 @@ static __always_inline unsigned long arch_local_irq_save(void)
 	     ".popsection")
 
 extern void default_banner(void);
+void native_pv_lock_init(void) __init;
 
 #else  /* __ASSEMBLY__ */
 
@@ -776,8 +777,13 @@ extern void default_banner(void);
 #endif /* __ASSEMBLY__ */
 #else  /* CONFIG_PARAVIRT */
 # define default_banner x86_init_noop
+
+static inline void native_pv_lock_init(void)
+{
+}
 #endif /* !CONFIG_PARAVIRT */
 
 #ifndef __ASSEMBLY__
diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
index d87451df480bd..cde8357bb226d 100644
--- a/arch/x86/include/asm/qspinlock.h
+++ b/arch/x86/include/asm/qspinlock.h
@@ -74,8 +74,6 @@ static inline bool vcpu_is_preempted(long cpu)
  */
 DECLARE_STATIC_KEY_TRUE(virt_spin_lock_key);
 
-void native_pv_lock_init(void) __init;
-
 /*
  * Shortcut for the queued_spin_lock_slowpath() function that allows
  * virt to hijack it.
@@ -103,10 +101,7 @@ static inline bool virt_spin_lock(struct qspinlock *lock)
 
 	return true;
 }
-#else
-static inline void native_pv_lock_init(void)
-{
-}
+
 #endif /* CONFIG_PARAVIRT */
 
 #include <asm-generic/qspinlock.h>
diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
index 89842bb7ec9cc..066fc19d2568e 100644
--- a/arch/x86/kernel/paravirt.c
+++ b/arch/x86/kernel/paravirt.c
@@ -75,7 +75,8 @@ DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key);
 
 void __init native_pv_lock_init(void)
 {
-	if (!boot_cpu_has(X86_FEATURE_HYPERVISOR))
+	if (IS_ENABLED(CONFIG_PARAVIRT_SPINLOCKS) &&
+	    !boot_cpu_has(X86_FEATURE_HYPERVISOR))
 		static_branch_disable(&virt_spin_lock_key);
 }