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

From: Cédric Le Goater
Subject: Re: [Qemu-ppc] [PATCH v2 02/19] spapr: introduce a skeleton for the XIVE interrupt controller
Date: Wed, 17 Jan 2018 10:18:43 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2

>>> 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.


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.

>>> 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. 

>>> 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.

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