Re: [RFC][PATCH 5/5] x86/kvm: Simplify static call handling

[Sean Christopherson <seanjc@google.com>]

"KVM: x86:" please, "x86/kvm" is for guest-side changes.

On Fri, Mar 10, 2023, Josh Poimboeuf wrote:
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 1dfba499d3e5..612531e1c478 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1789,8 +1789,6 @@ extern struct kvm_x86_ops kvm_x86_ops;
>  
>  #define KVM_X86_OP(func) \
>  	DECLARE_STATIC_CALL(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>
>  
>  int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops);
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index 6accb46295a3..5f7f860c5f17 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -77,20 +77,15 @@ static struct kvm_pmu_ops kvm_pmu_ops __read_mostly;
>  #define KVM_X86_PMU_OP(func)					     \
>  	DEFINE_STATIC_CALL_NULL(kvm_x86_pmu_##func,			     \
>  				*(((struct kvm_pmu_ops *)0)->func));
> -#define KVM_X86_PMU_OP_OPTIONAL KVM_X86_PMU_OP
>  #include <asm/kvm-x86-pmu-ops.h>
>  
>  void kvm_pmu_ops_update(const struct kvm_pmu_ops *pmu_ops)
>  {
>  	memcpy(&kvm_pmu_ops, pmu_ops, sizeof(kvm_pmu_ops));
>  
> -#define __KVM_X86_PMU_OP(func) \
> -	static_call_update(kvm_x86_pmu_##func, kvm_pmu_ops.func);
>  #define KVM_X86_PMU_OP(func) \
> -	WARN_ON(!kvm_pmu_ops.func); __KVM_X86_PMU_OP(func)

I would much prefer to keep KVM mostly as-is, specifically so that we don't lose
this WARN_ON() that guards against a vendor module neglecting to implement a
mandatory callback.  This effectively gives KVM "full" protection against consuming
an unexpectedly-NULL function pointer.

It's not strictly necessary, but I'm inclinded to keep KVM_X86_OP_OPTIONAL_RET0
around for documentation purposes.  And if I could figure out the compiler magic
to make it work, a WARN/BUILD_BUG on the sizeof() the return type of a RET0
function being larger than sizeof(int).  KVM encountered the fun that occurs on
a u64 return on 32-bit kernels due to "xor eax, eax" leaving garbage in edx.