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Re: [Qemu-ppc] [PATCH v2 02/19] spapr: introduce a skeleton for the XIVE

From: David Gibson
Subject: Re: [Qemu-ppc] [PATCH v2 02/19] spapr: introduce a skeleton for the XIVE interrupt controller
Date: Thu, 12 Apr 2018 15:10:31 +1000
User-agent: Mutt/1.9.2 (2017-12-15)

On Wed, Jan 17, 2018 at 10:18:43AM +0100, Cédric Le Goater wrote:
> >>> Also, have we decided how the process of switching between XICS and
> >>> XIVE will work vs. CAS ? 
> >>
> >> That's how it is described in the architecture. The current choice is
> >> to create both XICS and XIVE objects and choose at CAS which one to
> >> use. It relies today on the capability of the pseries machine to 
> >> allocate IRQ numbers for both interrupt controller backends. These
> >> patches have been merged in QEMU.
> >>
> >> A change of interrupt mode results in a reset. The device tree is 
> >> populated accordingly and the ICPs are switched for the model in 
> >> use. 
> > 
> > For KVM we need to only instanciate one of them though.
> Hmm,
> How would we handle a guest rebooting on a kernel without XIVE support ? 
> Are you suggesting to create the XICS or XIVE device in the CAS negotiation 
> process ? So, the machine would not have any interrupt controller before 
> CAS. That seems really late to me. grub uses the console for instance. 
> I think it should prepare for both options, start in XIVE legacy mode, 
> which is XICS, then possibly switch to XIVE exploitation mode.

I think for our first draft we should have XIVE and XICS based
platforms as separate machine types (or a machine option, I guess).

We do want to allow this to be autonegotiated, but I feel like
emphasising that at the beginning is causing unnatural design
decisions in the XIVE model itself.

> >>> And how that will interact with KVM ?
> >>
> >> I expect we will do the same, which is to create two KVM devices to 
> >> be able to handle both interrupt controller backends depending on the 
> >> mode negotiated by the guest.  
> > 
> > That will be an ungodly mess, I'd rather we only instanciate the right
> > one.
> It's rather transparent currently in the emulated version. There are two 
> sets of objects in QEMU, switching is done in CAS. KVM support should not 
> change anything in that area. 
> I expect the 'xive-kvm' object to get/set states for migration, just like 
> for XICS and to setup the ESB+TIMA memory regions, which is new. 
> C. 
> >>> I was
> >>> thinking the kernel would implement a different KVM device type, ie
> >>> the "emulated XICS" would remain KVM_DEV_TYPE_XICS and XIVE would be
> >>
> >> yes. it makes sense. The new device will have a lot in common with the 
> >> KVM_DEV_TYPE_XICS using kvm_xive_ops.
> > 
> > Ben.
> > 

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_!

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