On Fri, 15 Jun 2018, Ricardo Neri wrote:

On Fri, Jun 15, 2018 at 12:29:06PM +0200, Thomas Gleixner wrote:

quoted

You have to consider two cases:

 1) !remapped mode:

    That's reasonably simple because you just have to deal with the HPET
    TIMERn_PROCMSG_ROUT register. But then you need to do this directly and
    not through any of the existing interrupt facilities.

Indeed, there is no need to use the generic interrupt faciities to set affinity;
I am dealing with an NMI anyways.

quoted

 2) remapped mode:

    That's way more complex as you _cannot_ ever do anything which touches
    the IOMMU and the related tables.

    So you'd need to reserve an IOMMU remapping entry for each CPU upfront,
    store the resulting value for the HPET TIMERn_PROCMSG_ROUT register in
    per cpu storage and just modify that one from NMI.

    Though there might be subtle side effects involved, which are related to
    the acknowledge part. You need to talk to the IOMMU wizards first.

I see. I will look into the code and prototype something that makes sense for
the IOMMU maintainers.

I'd recommend to talk to them _before_ you cobble something together. If we
cannot reliably switch the affinity by directing the HPET NMI to a
different IOMMU remapping entry then the whole scheme does not work at all.

Thanks,

	tglx