Re: [PATCH] x86: fix off-by-one in nr_irqs_gsi calculation

[Keir Fraser <keir.xen@gmail.com>]

On 26/07/2012 16:43, "Jan Beulich" <JBeulich@suse.com> wrote:

>>>> On 26.07.12 at 17:31, Keir Fraser <keir@xen.org> wrote:
>> On 26/07/2012 16:06, "Jan Beulich" <JBeulich@suse.com> wrote:
>> 
>>> highest_gsi() returns the last valid GSI, not a count.
>>> 
>>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>> 
>> Why not "nr_irqs_gsi = max(nr_irqs_gsi, highest_gsi()+1)"?
> 
> While I think x = max(y, z) is fine, I generally find x = max(x, y)
> pretty useless - what's the point of assigning a value to itself?
> This is __init code, so code size considerations probably don't
> matter, but I'd still like to not encourage inefficient code like this
> to be used elsewhere by giving a bad example...

I think the code is clearer with max(). An even stronger argument is that it
makes the intent of the patch much more obvious too. I think you should
leave the max() construct in place in this patch.

 -- Keir

> Jan
> 
>>> --- a/xen/arch/x86/io_apic.c
>>> +++ b/xen/arch/x86/io_apic.c
>>> @@ -2531,7 +2531,9 @@ void __init init_ioapic_mappings(void)
>>>          }
>>>      }
>>>  
>>> -    nr_irqs_gsi = max(nr_irqs_gsi, highest_gsi());
>>> +    i = highest_gsi();
>>> +    if ( i >= nr_irqs_gsi )
>>> +        nr_irqs_gsi = i + 1;
>>>  
>>>      if ( max_gsi_irqs == 0 )
>>>          max_gsi_irqs = nr_irqs ? nr_irqs / 8 : PAGE_SIZE;
>>> 
>>> 
>>> 
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> 
> 
>