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

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Wed, 2018-01-17 at 10:18 +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 ? 

It will do CAS again and we can change the devices.

> 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. 

We start with XICS by default.

> I think it should prepare for both options, start in XIVE legacy mode, 
> which is XICS, then possibly switch to XIVE exploitation mode.
> 
> > > > 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. 

But both XICS and XIVE are completely different kernel KVM devices that will
need to "hook" into the same set of internal hooks for things like interrupts
being passed through, RTAS calls etc... 

How does KVM knows which one to "activate" ?

I don't think the kernel should have both. 

> > > > 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
> > > > KVM_DEV_TYPE_XIVE.
> > > 
> > > yes. it makes sense. The new device will have a lot in common with the 
> > > KVM_DEV_TYPE_XICS using kvm_xive_ops.
> > 
> > Ben.
> >