[Xen-users] Disabling specific non-sequential CPUs?

["E. Westbrook" <xen@xxxxxxxxxx>]

One of the six cores on one of my i7-3960x machines went bad some time ago.  
Activity on CPUs 2 and 8 resets the system -- somewhat unpredictably, but 
eventually, and invariably.

Since then, I've been running that machine's dom0 without Xen as standalone 
linux (3.19.3).  Linux very faithfully leaves those CPUs alone, as long as this 
gets done very early in the boot:

  for i in 2 8 ; do echo 0 > /sys/devices/system/cpu/cpu${i}/online ; done

Once that's done, the standalone linux cheerfully remains up indefinitely.

But now I can't avoid using Xen on this machine any longer.

I've mitigated the problem under Xen two ways:

(1) By adding the kernel argument "dom0_vcpus_pin", so that dom0 sees and 
disables the problem CPUs consistently the same way as when standalone, at 
least for dom0 itself.

(2) In my XL configs, I can also conveniently enough do

  cpus='0-1,3-7,9-11'

Which keeps the domUs themselves from taking things south.

These two mitigations delay the symptom considerably -- but eventually, at some 
point, Xen, which hasn't truly been told not to use those two CPUs, presumably 
and unsurprisingly does something on those CPUs itself, and, boom.

I've read everything I could find on Xen CPU pinning, masking, best practices, 
etc.  I remain stymied for an answer, so I now ask plaintively here for help.

To sum up the question:

How can I tell Xen to never, ever, do anything at all, nor let anything else do 
anything, with those two specific and non-sequential CPUs?

Thanks for any help!
Eric

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