Re: [PATCH] spapr/xive: Allocate IPIs from the vCPU contexts

[Cédric Le Goater]

On 8/20/20 2:42 AM, David Gibson wrote:
> On Sun, Aug 16, 2020 at 03:38:20PM +0200, Cédric Le Goater wrote:
>> On 8/16/20 6:30 AM, David Gibson wrote:
>>> On Fri, Aug 14, 2020 at 05:08:13PM +0200, Cédric Le Goater wrote:
>>>>
>>>> This works as expected with a 128 vCPUs guest with pinned vcpus. The
>>>> first 64 IPIs are allocated on the first chip and the remaining 64
>>>> on the second chip.
>>>>
>>>> Still, this is more an RFC. We have time before the end of the merge
>>>> window.
>>>
>>> It looks reasonable to me.  AFAICT it makes things better than they
>>> were, and even if we can improve it further, that won't break
>>> migration or other interfaces we need to preserve.
>>
>> Yeah. What I don't like is this test below. I am not sure that 
>> machine->smp.cpus is the correct way to test the number of currently
>> active vCPUs.
> 
> Ah, yeah.  It should be correct at initial startup, but might not be
> after a bunch of hotplugs/unplugs, which could result in a sparse set
> of active vcpus.

Yes. I have a v2 ready to be sent in which the vCPU IPI is allocated
under kvmppc_xive_cpu_connect().

Thanks,

C. 

> 
> Usually this code will be called during initial setup, but I think
> there are some edge cases where it won't be (e.g. boot a XICS kernel,
> do some vcpu plugs/unplugs, reboot into a XIVE kernel).
> 
> So I think we need to explicitly check for each vcpu # if it's
> currently active.  Using... spapr_find_cpu(), I guess?
> 
>>
>>>>> +    if (srcno < machine->smp.cpus) {
>>>>> +        return kvmppc_xive_reset_ipi(xive, srcno, errp);
>>>>> +    }
>>>>> +
>>>>>      if (xive_source_irq_is_lsi(xsrc, srcno)) {
>>>>>          state |= KVM_XIVE_LEVEL_SENSITIVE;
>>>>>          if (xsrc->status[srcno] & XIVE_STATUS_ASSERTED) {
>>
>