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Re: [RFC PATCH] Use vAPIC when doing IPI for PVHVM guests.

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On 10/07/2015 10:21 PM, Konrad Rzeszutek Wilk wrote:
> Hey,
>
> I was running some tools in which we would heavily do rescheduling
> of events - and realized to my surprise that the event channels (and
> the hypercall) would slow things down. If I used the vAPIC with its
> IPI support (so no VMEXIT) I got much much better performance.
>
> Now this is an RFC because:
>   1). I hadn't verified from the xentrace  how much less VMEXITS we get.
>      But I remember Boris's patches and they gave at least 10%.
>      I think this will get the same performance or even better.
>
>   2). I don't know what to do with migration. That is if the guest
>      migrates to older hardware it needs to recheck this I presume?

Same problem applies to many other features. In case you want to
migrate to a machine with less features you'd have to mask those
features in the cpuid data of the domain.

>   3). Should this be enabled by default? I did get better performance
>     but that was synthetic.

Having some benchmark results would help to decide this. :-)

I'd be especially interested in checking "no vcpu over-commitment"
and "heavy vcpu over-commitment" scenarios regarding the effect of
the feature.

>
> Thoughts?

I like the idea.


Juergen
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[RFC PATCH] Use vAPIC when doing IPI for PVHVM guests. Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> - 2015-10-07 22:30 +0200
  [RFC PATCH v1 2/2] xen_nopv: Combine a bunch of the PV features that can be disabled Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> - 2015-10-07 22:30 +0200
    Re: [RFC PATCH v1 2/2] xen_nopv: Combine a bunch of the PV features  that can be disabled Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2015-10-07 23:40 +0200
  Re: [RFC PATCH] Use vAPIC when doing IPI for PVHVM guests. Juergen Gross <jgross@suse.com> - 2015-10-08 07:10 +0200
    Re: [Xen-devel] [RFC PATCH] Use vAPIC when doing IPI for PVHVM guests. Andrew Cooper <andrew.cooper3@citrix.com> - 2015-10-08 11:40 +0200

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