linux-kernel - Re: [PATCH] KVM: update depracated jump label API

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Date:   Mon, 11 Jan 2021 18:15:43 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Cun Li <cun.jia.li@...il.com>
Cc:     seanjc@...gle.com, wanpengli@...cent.com, jmattson@...gle.com,
        joro@...tes.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Cun Li <cun.jia.li@...il.com>,
        pbonzini@...hat.com
Subject: Re: [PATCH] KVM: update depracated jump label API

Cun Li <cun.jia.li@...il.com> writes:

> The use of 'struct static_key' and 'static_key_false' is
> deprecated. Use the new API.
>
> Signed-off-by: Cun Li <cun.jia.li@...il.com>
> ---
>  arch/x86/kvm/lapic.h         | 6 +++---
>  arch/x86/kvm/mmu/mmu_audit.c | 4 ++--
>  arch/x86/kvm/x86.c           | 2 +-
>  3 files changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
> index 4fb86e3a9dd3..b7aa76e2678e 100644
> --- a/arch/x86/kvm/lapic.h
> +++ b/arch/x86/kvm/lapic.h
> @@ -176,7 +176,7 @@ extern struct static_key kvm_no_apic_vcpu;
>  
>  static inline bool lapic_in_kernel(struct kvm_vcpu *vcpu)
>  {
> -	if (static_key_false(&kvm_no_apic_vcpu))
> +	if (static_branch_unlikely(&kvm_no_apic_vcpu))
>  		return vcpu->arch.apic;
>  	return true;
>  }
> @@ -185,7 +185,7 @@ extern struct static_key_deferred apic_hw_disabled;
>  
>  static inline int kvm_apic_hw_enabled(struct kvm_lapic *apic)
>  {
> -	if (static_key_false(&apic_hw_disabled.key))
> +	if (static_branch_unlikely(&apic_hw_disabled.key))
>  		return apic->vcpu->arch.apic_base & MSR_IA32_APICBASE_ENABLE;
>  	return MSR_IA32_APICBASE_ENABLE;
>  }
> @@ -194,7 +194,7 @@ extern struct static_key_deferred apic_sw_disabled;
>  
>  static inline bool kvm_apic_sw_enabled(struct kvm_lapic *apic)
>  {
> -	if (static_key_false(&apic_sw_disabled.key))
> +	if (static_branch_unlikely(&apic_sw_disabled.key))
>  		return apic->sw_enabled;
>  	return true;
>  }
> diff --git a/arch/x86/kvm/mmu/mmu_audit.c b/arch/x86/kvm/mmu/mmu_audit.c
> index c8d51a37e2ce..8a4b3510151a 100644
> --- a/arch/x86/kvm/mmu/mmu_audit.c
> +++ b/arch/x86/kvm/mmu/mmu_audit.c
> @@ -234,7 +234,7 @@ static void audit_vcpu_spte(struct kvm_vcpu *vcpu)
>  }
>  
>  static bool mmu_audit;
> -static struct static_key mmu_audit_key;
> +static DEFINE_STATIC_KEY_FALSE(mmu_audit_key);
>  
>  static void __kvm_mmu_audit(struct kvm_vcpu *vcpu, int point)
>  {
> @@ -250,7 +250,7 @@ static void __kvm_mmu_audit(struct kvm_vcpu *vcpu, int point)
>  
>  static inline void kvm_mmu_audit(struct kvm_vcpu *vcpu, int point)
>  {
> -	if (static_key_false((&mmu_audit_key)))
> +	if (static_branch_unlikely((&mmu_audit_key)))
>  		__kvm_mmu_audit(vcpu, point);
>  }
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9a8969a6dd06..b8c05ef26942 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10339,7 +10339,7 @@ bool kvm_vcpu_is_bsp(struct kvm_vcpu *vcpu)
>  	return (vcpu->arch.apic_base & MSR_IA32_APICBASE_BSP) != 0;
>  }
>  
> -struct static_key kvm_no_apic_vcpu __read_mostly;
> +__read_mostly DEFINE_STATIC_KEY_FALSE(kvm_no_apic_vcpu);
>  EXPORT_SYMBOL_GPL(kvm_no_apic_vcpu);
>  
>  void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu)

mmu_audit_key can only be true or false so it would also be nice to use 
static_branch_enable()/static_branch_disable() for it and not
static_key_slow_inc()/static_key_slow_dec() we currently use (as it
sounds weird to increment 'false').

kvm_no_apic_vcpu is different, we actually need to increase it with
every vCPU which doesn't have LAPIC but maybe we can at least switch to
static_branch_inc()/static_branch_dec(). It is still weird we initialize
it to 'false' but it seems to be a documented behavior. From
include/linux/jump_label.h:

"... Thus, static_branch_inc() can be thought of as a 'make more true'
 and static_branch_dec() as a 'make more false'"

so .. oh well.

-- 
Vitaly