Re: [Xen-users] Xen dom0 vCPUs pinning for PV/HVM domUs

[Dario Faggioli <dfaggioli@xxxxxxxx>]

On Wed, 2018-02-14 at 15:37 +0100, John Naggets wrote:
> Thank you Dario for your insight. Indeed I now find the
> dom0_vcpus_ping really confusing. In the wiki documentation it says:
> 
> "Another interesting approach is pinning Dom0 vCPUs to physical CPUs,
> this can be done by adding dom0_vcpus_pin to the Xen command line.
> Then once Dom0 has booted you can see to which CPUs the vCPUs have
> been pinned and exclude other domains from running on those CPUs."
> 
> Source: https://wiki.xenproject.org/wiki/Tuning_Xen_for_Performance#D
> om0_vCPUs
> 
> I think then this documentation should be adapted to reflect the
> reality and not inducing people into confusion.
> 
What is said there is correct.

And, in the following paragraph, it is also said how to actually
achieve dom0/domU pCPUs isolation. What I don't like about the
dom0_vcpus_pin is:
- it's very inflexible. I.e., it does not allow to say anything like 
  "pin dom0's vCPUs to pCPUs x,y,z-w". Instead, it will always pin 
  d0v0 to pCPU 0, d0v1 to pCPU 1, d0v2 to pCPU2, etc;
- IIRC, if you boot with dom0_vcpus_pin, you can't change dom0 vCPUs
  affinity (and I never understood why this has been made to be the
  case...)
- people may see it and tend to think that, just by using it, they'll 
  achieve dom0 isolation, without bothering going reading that page, 
  and actually finding out that they also need to put something in 
  _all_ their domUs' configs;
- I do find it very impractical to have to put something in _all_ the 
  domUs' configs, and although I appreciate that someone may be 
  willing to do that, and am fine with it, I think we should not 
  encourage doing it.

So, although that page does reflect the reality, I agree that it could
be restructured a bit.

Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
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Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Software Engineer @ SUSE https://www.suse.com/

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