[RFC PATCH v1] powerpc/prom_init: disable XIVE in Secure VM.

[David Gibson]

On Fri, Feb 28, 2020 at 11:54:04PM -0800, Ram Pai wrote:
> XIVE is not correctly enabled for Secure VM in the KVM Hypervisor yet.
> 
> Hence Secure VM, must always default to XICS interrupt controller.
> 
> If XIVE is requested through kernel command line option "xive=on",
> override and turn it off.
> 
> If XIVE is the only supported platform interrupt controller; specified
> through qemu option "ic-mode=xive", simply abort. Otherwise default to
> XICS.

Uh... the discussion thread here seems to have gotten oddly off
track.  So, to try to clean up some misunderstandings on both sides:

  1) The guest is the main thing that knows that it will be in secure
     mode, so it's reasonable for it to conditionally use XIVE based
     on that.

  2) The mechanism by which we do it here isn't quite right.  Here the
     guest is checking itself that the host only allows XIVE, but we
     can't do XIVE and is panic()ing.  Instead, in the SVM case we
     should force support->xive to false, and send that in the CAS
     request to the host.  We expect the host to just terminate
     us because of the mismatch, but this will interact better with
     host side options setting policy for panic states and the like.
     Essentially an SVM kernel should behave like an old kernel with
     no XIVE support at all, at least w.r.t. the CAS irq mode flags.

  3) Although there are means by which the hypervisor can kind of know
     a guest is in secure mode, there's not really an "svm=on" option
     on the host side.  For the most part secure mode is based on
     discussion directly between the guest and the ultravisor with
     almost no hypervisor intervention.

  4) I'm guessing the problem with XIVE in SVM mode is that XIVE needs
     to write to event queues in guest memory, which would have to be
     explicitly shared for secure mode.  That's true whether it's KVM
     or qemu accessing the guest memory, so kernel_irqchip=on/off is
     entirely irrelevant.

  5) All the above said, having to use XICS is pretty crappy.  You
     should really get working on XIVE support for secure VMs.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson
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