Re: [PATCH v2 11/13] KVM: x86: add KVM_CAP_X2APIC_API

Yang Zhang
2016-07-11T10:33:49Z

On 2016/7/11 17:17, Paolo Bonzini wrote:
>
>
> On 11/07/2016 10:56, Yang Zhang wrote:
>> On 2016/7/11 15:44, Paolo Bonzini wrote:
>>>
>>>
>>> On 11/07/2016 08:06, Yang Zhang wrote:
>>>>> Changes to MSI addresses follow the format used by interrupt remapping
>>>>> unit.
>>>>> The upper address word, that used to be 0, contains upper 24 bits of
>>>>> the LAPIC
>>>>> address in its upper 24 bits. Lower 8 bits are reserved as 0.
>>>>> Using the upper address word is not backward-compatible either as we
>>>>> didn't
>>>>> check that userspace zeroed the word. Reserved bits are still not
>>>>> explicitly
>>>>
>>>> Does this means we cannot migrate the VM from KVM_CAP_X2APIC_API enabled
>>>> host to the disable host even VM doesn't have more than 255 VCPUs?
>>>
>>> Yes, but that's why KVM_CAP_X2APIC_API is enabled manually. The idea is
>>> that QEMU will not use KVM_CAP_X2APIC_API except on the newest machine
>>> type.
>>
>> Thanks for confirmation. And when the KVM_CAP_X2APIC_API will be enabled
>> in Qemu?
>
> It could be 2.7 or 2.8.
>
>>>
>>> If interrupt remapping is on, KVM_CAP_X2APIC_API is needed even with 8
>>> VCPUs, I think. Otherwise KVM will believe that 0xff is "broadcast"
>>> rather than "cluster 0, CPUs 0-7".
>>
>> If interrupt remapping is using, what 0xff means is relying on which
>> mode the destination CPU is in. I think there is no KVM_CAP_X2APIC_API
>> needed since interrupt remapping table gives all the information.
>
> If you have EIM 0xff never means broadcast, but KVM sees a 0xff in the
> interrupt route or KVM_SIGNAL_MSI argument and translates it into a
> broadcast.

I see your point. I thought there would be a new irq router(like
KVM_IRQ_ROUTING_IR) to handle all interrupts after turn on IR and
KVM_CAP_X2APIC_API would be dropped. So we will continue to use
KVM_IRQ_ROUTING_IRQCHIP and KVM_IRQ_ROUTING_MSI for interrupt from IR,
right?

--
best regards
yang