linux-kernel - Re: [PATCH v3 6/6] KVM: x86: allow defining return-0 static calls

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Date:   Fri, 18 Feb 2022 16:29:25 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v3 6/6] KVM: x86: allow defining return-0 static calls

On Thu, Feb 17, 2022, Paolo Bonzini wrote:
> A few vendor callbacks are only used by VMX, but they return an integer
> or bool value.  Introduce KVM_X86_OP_OPTIONAL_RET0 for them: if a func is
> NULL in struct kvm_x86_ops, it will be changed to __static_call_return0
> when updating static calls.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/include/asm/kvm-x86-ops.h | 15 +++++++++------
>  arch/x86/include/asm/kvm_host.h    |  4 ++++
>  arch/x86/kvm/svm/avic.c            |  5 -----
>  arch/x86/kvm/svm/svm.c             | 20 --------------------
>  arch/x86/kvm/x86.c                 |  2 +-
>  kernel/static_call.c               |  1 +
>  6 files changed, 15 insertions(+), 32 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> index c0ec066a8599..29affccb353c 100644
> --- a/arch/x86/include/asm/kvm-x86-ops.h
> +++ b/arch/x86/include/asm/kvm-x86-ops.h
> @@ -10,7 +10,9 @@ BUILD_BUG_ON(1)
>   *
>   * KVM_X86_OP_OPTIONAL() can be used for those functions that can have
>   * a NULL definition, for example if "static_call_cond()" will be used
> - * at the call sites.
> + * at the call sites.  KVM_X86_OP_OPTIONAL_RET0() can be used likewise
> + * to make a definition optional, but in this case the default will
> + * be __static_call_return0.

__static_call_return0()

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ab1c4778824a..d3da64106685 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -131,6 +131,7 @@ struct kvm_x86_ops kvm_x86_ops __read_mostly;
>  	DEFINE_STATIC_CALL_NULL(kvm_x86_##func,			     \
>  				*(((struct kvm_x86_ops *)0)->func));
>  #define KVM_X86_OP_OPTIONAL KVM_X86_OP
> +#define KVM_X86_OP_OPTIONAL_RET0 KVM_X86_OP
>  #include <asm/kvm-x86-ops.h>
>  EXPORT_STATIC_CALL_GPL(kvm_x86_get_cs_db_l_bits);
>  EXPORT_STATIC_CALL_GPL(kvm_x86_cache_reg);
> @@ -12016,7 +12017,6 @@ void kvm_arch_flush_shadow_memslot(struct kvm *kvm,
>  static inline bool kvm_guest_apic_has_interrupt(struct kvm_vcpu *vcpu)
>  {
>  	return (is_guest_mode(vcpu) &&
> -			kvm_x86_ops.guest_apic_has_interrupt &&
>  			static_call(kvm_x86_guest_apic_has_interrupt)(vcpu));

Can you opportunistically align the indentation and drop the outer parantheses? I.e.

	return is_guest_mode(vcpu) &&
	       static_call(kvm_x86_guest_apic_has_interrupt)(vcpu);

>  }
>  
> diff --git a/kernel/static_call.c b/kernel/static_call.c
> index 43ba0b1e0edb..76abd46fe6ee 100644
> --- a/kernel/static_call.c
> +++ b/kernel/static_call.c
> @@ -503,6 +503,7 @@ long __static_call_return0(void)
>  {
>  	return 0;
>  }
> +EXPORT_SYMBOL_GPL(__static_call_return0)

This doesn't compile, it needs a semicolon.