Re: [PATCH] kvm: x86: lapic: remove one redundant judging condition

[Paolo Bonzini <pbonzini@redhat.com>]

On 05/11/2014 10:03, Tiejun Chen wrote:
> Finally we always return highest_irr so its unnecessary to return -1
> after check if highest_irr == -1.
> 
> Signed-off-by: Tiejun Chen <tiejun.chen@intel.com>
> ---
>  arch/x86/kvm/lapic.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 5f574b4..e6a7eb6 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1638,8 +1638,7 @@ int kvm_apic_has_interrupt(struct kvm_vcpu *vcpu)
>  
>  	apic_update_ppr(apic);
>  	highest_irr = apic_find_highest_irr(apic);
> -	if ((highest_irr == -1) ||
> -	    ((highest_irr & 0xF0) <= kvm_apic_get_reg(apic, APIC_PROCPRI)))
> +	if ((highest_irr & 0xF0) <= kvm_apic_get_reg(apic, APIC_PROCPRI))
>  		return -1;
>  	return highest_irr;
>  }

I think the code is clearer without this change.

The two returns mean:

- return -1: no interrupt to inject

- return highest_irr: inject this interrupt

With IRR equal to all zeroes (highest_irr = -1), your patch would make 
the "if" always false ("current PPR is low, can inject the interrupt"), 
but computing highest_irr & 0xF0 would make no sense if highest_irr == 
-1.

To put it another way, imagine the code looked like this:

static inline int int_prio(int vector)
{
	WARN_ON(vector == -1);
	return vector & 0xF0;
}
...

 	apic_update_ppr(apic);
 	highest_irr = apic_find_highest_irr(apic);
	if (highest_irr == -1 ||
	    int_prio(highest_irr) <= kvm_apic_get_reg(apic, APIC_PROCPRI))
 		return -1;
 	return highest_irr;

Then removing the check on highest_irr == -1 would trigger a warning.

Paolo