2017-11-22 18:28-0200, Eduardo Habkost: > On Wed, Nov 22, 2017 at 07:09:08PM +0100, Radim Krčmář wrote: > > QEMU saves only 8 bits of APIC LDR, which means that it does not support > > x2APIC. The correct way of fixing this would be to save and restore the > > full 32 bit register, but because x2APIC LDR is a function of x2APIC ID, > > we can also compute it and keep the migration format untouched. > > > > KVM always expected the LDR format to follow the xAPIC/x2APIC standard, > > but pre 4.1 KVMs used non-standard x2APIC ID in case the OS changed > > xAPIC ID before switching to x2APIC, which means that QEMU has to use > > the kvm_x2apic_api feature to derive the x2APIC ID. > > > > This bug has also been addressed on the KVM side with patch 5849d75a5c9b > > ("KVM: lapic: Fixup LDR on load in x2apic"). > > Is this sufficient to fix the bug on hosts that lack KVM commit > 5849d75a5c9b, or we need both the KVM and QEMU patches?
Good point, either one is enough to fix the bug. This patch makes x2APIC LDR work on old KVMs.