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Re: [Qemu-ppc] [PATCH v2 02/19] spapr: introduce a skeleton for the XIVE
Cédric Le Goater
Re: [Qemu-ppc] [PATCH v2 02/19] spapr: introduce a skeleton for the XIVE interrupt controller
Thu, 12 Apr 2018 10:41:21 +0200
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On 04/12/2018 07:10 AM, David Gibson wrote:
> 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
>>> 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.
> I think for our first draft we should have XIVE and XICS based
> platforms as separate machine types (or a machine option, I guess).
OK. This is my current choice for KVM.
Emulated mode is rather simple to handle, and this is why I have
kept the reset after CAS if there is a change in the interrupt mode.
> 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.
Yes. This is mostly a KVM problem which also has impacts on XICS
of course ...
>>>>> 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
>> 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.