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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 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).

> 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.


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