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Re: [Xen-devel] [PATCH v2] x86/apic/x2apic: Share IRQ vector between cluster members only when no cpumask is specified

>>> On 28.08.17 at 16:35, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 08/28/2017 03:38 AM, Jan Beulich wrote:
>>>> And finally I continue to be not really happy about the change as
>>>> a whole. Despite what was discussed on v1, I'm concerned of the
>>>> effects of this on hosts _not_ suffering from vector shortage.
>>>> Could you live with the new behavior requiring a command line
>>>> option to enable?
>>> I can add something like 'apic_share_vectors', defaulting to true,
>>> although it will not be useful in case of a hotplug. Defaulting to false?
>> Along the lines of the above plus our desire to limit the number
>> of top level options, how about "apic=isolate-vectors"?
>> Also I don't understand your reference to hotplug, nor why you
>> suggest two opposite default values.
> Not two, just one --- not share vectors by default.
> As for hotplug, I was thinking of a case where a system is successfully
> booted with shared vectors but then a device is added and we fail to
> find enough free vectors. So the administrator would need to know in
> advance whether a new card might be coming in.
> When defaulting to false (as in apic_share_vectors=false) if the
> administrator decides to set it to true then he would be in some sense
> explicitly agreeing to never plug anything in (or at least to tolerate
> such a failure).

Ah, I see. But imo the default ought to be current behavior.

>> But finally, you agreeing to a command line option here makes
>> me come back to an earlier suggestion: Didn't we agree that
>> "x2apic_phys" would be sufficient to eliminate the issue? In which
>> case no patch would be needed at all.
> x2apic_phys means that we never share vectors. With 'isolate-vectors'
> option we are still able to share them if the mask is explicitly specified.

Well, aiui your primary goal is to address the vector shortage. For
that you don't need the new option, you can get away with the
existing one.


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