Re: [Qemu-devel] [PATCH v3 04/35] spapr/xive: introduce a XIVE interrupt controller for sPAPR

["Cédric Le Goater" <clg@kaod.org>]

On 05/05/2018 06:26 AM, David Gibson wrote:
> On Fri, May 04, 2018 at 03:05:08PM +0200, Cédric Le Goater wrote:
>> On 05/04/2018 05:33 AM, David Gibson wrote:
>>> On Thu, May 03, 2018 at 06:50:09PM +0200, Cédric Le Goater wrote:
>>>> On 05/03/2018 07:22 AM, David Gibson wrote:
>>>>> On Thu, Apr 26, 2018 at 12:43:29PM +0200, Cédric Le Goater wrote:
>>>>>> On 04/26/2018 06:20 AM, David Gibson wrote:
>>>>>>> On Tue, Apr 24, 2018 at 11:46:04AM +0200, Cédric Le Goater wrote:
>>>>>>>> On 04/24/2018 08:51 AM, David Gibson wrote:
>>>>>>>>> On Thu, Apr 19, 2018 at 02:43:00PM +0200, Cédric Le Goater wrote:
>>>>>>>>>> sPAPRXive is a model for the XIVE interrupt controller device of the
>>>>>>>>>> sPAPR machine. It holds the routing XIVE table, the Interrupt
>>>>>>>>>> Virtualization Entry (IVE) table which associates interrupt source
>>>>>>>>>> numbers with targets.
>>>>>>>>>>
>>>>>>>>>> Also extend the XiveFabric with an accessor to the IVT. This will be
>>>>>>>>>> needed by the routing algorithm.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Cédric Le Goater <clg@kaod.org>
>>>>>>>>>> ---
>>>>>>>>>>
>>>>>>>>>>  May be should introduce a XiveRouter model to hold the IVT. To be
>>>>>>>>>>  discussed.
>>>>>>>>>
>>>>>>>>> Yeah, maybe.  Am I correct in thinking that on pnv there could be more
>>>>>>>>> than one XiveRouter?
>>>>>>>>
>>>>>>>> There is only one, the main IC. 
>>>>>>>
>>>>>>> Ok, that's what I thought originally.  In that case some of the stuff
>>>>>>> in the patches really doesn't make sense to me.
>>>>>>
>>>>>> well, there is one IC per chip on powernv, but we haven't reach that part
>>>>>> yet.
>>>>>
>>>>> Hmm.  There's some things we can delay dealing with, but I don't think
>>>>> this is one of them.  I think we need to understand how multichip is
>>>>> going to work in order to come up with a sane architecture.  Otherwise
>>>>> I fear we'll end up with something that we either need to horribly
>>>>> bastardize for multichip, or have to rework things dramatically
>>>>> leading to migration nightmares.
>>>>
>>>> So, it is all controlled by MMIO, so we should be fine on that part. 
>>>> As for the internal tables, they are all configured by firmware, using
>>>> a chip identifier (block). I need to check how the remote XIVE are 
>>>> accessed. I think this is by MMIO. 
>>>
>>> Right, but for powernv we execute OPAL inside the VM, rather than
>>> emulating its effects.  So we still need to model the actual hardware
>>> interfaces.  OPAL hides the details from the kernel, but not from us
>>> on the other side.
>>
>> Yes. This is the case in the current model. I took a look today and
>> I have a few fixes for the MMIO layout for P9 chips which I will send.
>>
>> As for XIVE, the model needs to be a little more  complex to support 
>> VSD_MODE_FORWARD tables which describe how to forward a notification
>> to another XIVE IC on another chip. They contain an address on which 
>> to load, This is another hop in the notification chain.  
> 
> Ah, ok.  So is that mode and address configured in the (bare metal)
> IVT as well?  Or is that a different piece of configuration?

The mode of a virtual structure table is configured by firmware. 
There are 4 main table types:  IVT, SBE, EQD, VPD (and an extra one
for IRQ) for the 16 possible blocks of a machine (I am simplifying 
a bit there). 

Local tables to a block/chip, today, are set to EXCLUSIVE and all 
remotes tables set to FORWARD.

The address of a table is configured by FW also. In case of a 
FORWARD table, it is set to the remote IC BAR + one page. This page 
has two 2K windows : one for for HW interrupt triggers and another 
one for to forward interrupts and for operation synchronization. 
>>>> I haven't looked at multichip XIVE support but I am not too worried as 
>>>> the framework is already in place for the machine.
>>>>  
>>>>>>>>> If we did have a XiveRouter, I'm not sure we'd need the XiveFabric
>>>>>>>>> interface, possibly its methods could just be class methods of
>>>>>>>>> XiveRouter.
>>>>>>>>
>>>>>>>> Yes. We could introduce a XiveRouter to share the ivt table between 
>>>>>>>> the sPAPRXive and the PnvXIVE models, the interrupt controllers of
>>>>>>>> the machines. Methods would provide way to get the ivt/eq/nvt
>>>>>>>> objects required for routing. I need to add a set_eq() to push the
>>>>>>>> EQ data.
>>>>>>>
>>>>>>> Hrm.  Well, to add some more clarity, let's say the XiveRouter is the
>>>>>>> object which owns the IVT.  
>>>>>>
>>>>>> OK. that would be a model with some state and not an interface.
>>>>>
>>>>> Yes.  For papr variant it would have the whole IVT contents as its
>>>>> state.  For the powernv, just the registers telling it where to find
>>>>> the IVT in RAM.
>>>>>
>>>>>>> It may or may not do other stuff as well.
>>>>>>
>>>>>> Its only task would be to do the final event routing: get the IVE,
>>>>>> get the EQ, push the EQ DATA in the OS event queue, notify the CPU.
>>>>>
>>>>> That seems like a lot of steps.  Up to push the EQ DATA, certainly.
>>>>> And I guess it'll have to ping an NVT somehow, but I'm not sure it
>>>>> should know about CPUs as such.
>>>>
>>>> For PowerNV, the concept could be generalized, yes. An NVT can 
>>>> contain the interrupt state of a logical server but the common 
>>>> case is baremetal without guests for QEMU and so we have a NVT 
>>>> per cpu. 
>>>
>>> Hmm.  We eventually want to support a kernel running guests under
>>> qemu/powernv though, right?  
>>
>> arg. an emulated hypervisor ! OK let's say this is a long term goal :) 
>>
>>> So even if we don't allow it right now,
>>> we don't want allowing that to require major surgery to our
>>> architecture.
>>
>> That I agree on. 
>>
>>>> PowerNV will have some limitation but we can make it better than 
>>>> today for sure. It boots.
>>>>
>>>> We can improve some of the NVT notification process, the way NVT 
>>>> are matched eventually. may be support remote engines if the
>>>> NVT is not local. I have not looked at the details.
>>>>
>>>>> I'm not sure at this stage what should own the EQD table.
>>>>
>>>> The EQDT is in RAM.
>>>
>>> Not for spapr, it's not.  
>>
>> yeah ok. It's in QEMU/KVM.
>>
>>> And even when it is in RAM, something needs
>>> to own the register that gives its base address.
>>
>> It's more complex than registers on powernv. There is a procedure
>> to define the XIVE tables using XIVE table descriptors which contain
>> their characteristics, size, indirect vs. indirect, local vs remote.
>> OPAL/skiboot defines all these to configure the HW, and the model
>> necessarily needs to support the same interface. This is the case
>> for a single chip.
> 
> Ah, ok.  So there's some sort of IVTD. 

These are called Virtual Structure table Descriptors (VSDs). Each
XIVE chip has an array of these.

> Also in RAM?  

Yes. But the vsd are just temporary structures to configure HW. 
What is important is the information it is holding : IVT, EQDT,
VPDT, etc. 

> Eventually there
> must be a register giving the base address of the IVTD, yes?

There are two registers to configure the table. One to set the 
table type and block, and one to set its VSD.

C.