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

[Yang Zhang]

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?

> 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.

--
best regards
yang