On 2/6/19 2:18 AM, David Gibson wrote:

On Wed, Feb 06, 2019 at 09:13:15AM +1100, Paul Mackerras wrote:

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On Tue, Feb 05, 2019 at 12:31:28PM +0100, Cédric Le Goater wrote:

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As for nesting, I suggest for the foreseeable future we stick to XICS
emulation in nested guests.

ok. so no kernel_irqchip at all. hmm. 

I was confused with what Paul calls 'XICS emulation'. It's not the QEMU
XICS emulated device but the XICS-over-XIVE KVM device, the KVM XICS 
device KVM uses when under a P9 processor. 

Actually there are two separate implementations of XICS emulation in
KVM.  The first (older) one is almost entirely a software emulation
but does have some cases where it accesses an underlying XICS device
in order to make some things faster (IPIs and pass-through of a device
interrupt to a guest).  The other, newer one is the XICS-on-XIVE
emulation that Ben wrote, which uses the XIVE hardware pretty heavily.
My patch was about making the the older code work when there is no
XICS available to the host.

Ah, right.  To clarify my earlier statements in light of this:

 * We definitely want some sort of kernel-XICS available in a nested
   guest.  AIUI, this is now accomplished, so, Yay!

 * Implementing the L2 XICS in terms of L1's PAPR-XIVE would be a
   bonus, but it's a much lower priority.

Yes. In this case, the L1 KVM-HV should not advertise KVM_CAP_PPC_IRQ_XIVE
to QEMU which will restrict CAS to the XICS only interrupt mode.

C.