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. 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". Paolo
