Re: [Xen-devel] [PATCH 5/6] x86/IRQ: re-use legacy vector ranges on APs

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.12.2019 15:34, Andrew Cooper wrote:
> On 20/12/2019 13:30, Jan Beulich wrote:
>> The legacy vectors have been actively used on CPU 0 only. CPUs not
>> sharing vector space with CPU 0 can easily re-use them, slightly
>> increasing the relatively scarce resource of total vectors available in
>> the system.
> 
> I suppose this technically depends on ExtINT messages never targeting
> CPUs other than 0.
> 
> Either way - I think its fine restriction to rely on.

And setup_local_APIC() arranges for this.

>> @@ -2617,7 +2616,7 @@ void __init init_ioapic_mappings(void)
>>  
>>      if ( nr_irqs == 0 )
>>          nr_irqs = cpu_has_apic ?
> 
> We should strip the !cpu_has_apic paths because they are obsolete in
> 64bit processors.  I guess this can wait for a future cleanup series.
> 
>> -                  max(16U + num_present_cpus() * NR_DYNAMIC_VECTORS,
>> +                  max(0U + num_present_cpus() * NR_DYNAMIC_VECTORS,
> 
> num_present_cpus() really can't be negative.  Neither can
> cpumask_weight().  With a bit of API cleanup, this 0U cast can be dropped.

Right, but that's for another day.

> However, given this is the only concerned, Acked-by: Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx>

Thanks!

Jan

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