linux-kernel - Re: [PATCH v2 18/21] arm64: KVM: Introduce hyp_alternate

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Date:	Mon, 1 Feb 2016 15:41:03 +0100
From:	Christoffer Dall <christoffer.dall@...aro.org>
To:	Marc Zyngier <marc.zyngier@....com>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCH v2 18/21] arm64: KVM: Introduce hyp_alternate_value helper

On Mon, Jan 25, 2016 at 03:53:52PM +0000, Marc Zyngier wrote:
> We already have hyp_alternate_select() to define a function pointer
> that gets changed by a kernel feature or workaround.
> 
> It would be useful to have a similar feature that resolves in a
> direct value, without requiring a function call. For this purpose,
> introduce hyp_alternate_value(), which returns one of two values
> depending on the state of the alternative.
> 
> Signed-off-by: Marc Zyngier <marc.zyngier@....com>
> ---
>  arch/arm64/kvm/hyp/hyp.h | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/arch/arm64/kvm/hyp/hyp.h b/arch/arm64/kvm/hyp/hyp.h
> index 44eaff7..dc75fdb 100644
> --- a/arch/arm64/kvm/hyp/hyp.h
> +++ b/arch/arm64/kvm/hyp/hyp.h
> @@ -144,6 +144,17 @@ typeof(orig) * __hyp_text fname(void)					\
>  	return val;							\
>  }
>  
> +#define hyp_alternate_value(fname, orig, alt, cond)			\
> +typeof(orig) __hyp_text fname(void)					\
> +{									\
> +	typeof(alt) val = orig;						\
> +	asm volatile(ALTERNATIVE("nop		\n",			\
> +				 "mov	%0, %1	\n",			\
> +				 cond)					\
> +		     : "+r" (val) : "r" ((typeof(orig))alt));		\
> +	return val;							\
> +}
> +
>  void __vgic_v2_save_state(struct kvm_vcpu *vcpu);
>  void __vgic_v2_restore_state(struct kvm_vcpu *vcpu);
>  
> -- 
> 2.1.4
> 
I'm really not convinced that this is more readable than simply defining
a function where needed.  Perhaps the thing that needs a definition is
the "asm volatile(ALTERNATIVE(...))" part?  I also don't see why any of
this is specific to KVM or Hyp ?

-Christoffer