Linux 4.8 introduces changes that really really require you to set
the APIC base (via either KVM_SET_SREGS or KVM_SET_MSRS) before the
KVM_SET_LAPIC ioctl, unless KVM_SET_LAPIC is immediately preceded by
KVM_GET_LAPIC.  This is because in x2apic mode KVM now moves the APIC id
from bits 31-24 of the KVM_SET_LAPIC argument to bits 7-0 of the APIC_ID
register (0x20).  If QEMU does not nudge the APIC out of x2apic at reset
time, the subsequent KVM_SET_LAPIC incorrectly shifts the APIC id right
and Linux sees (in bits 31-24) a zero id.

While QEMU has always done the right thing, the recent commit 78d6a05
("x86/lapic: Load LAPIC state at post_load", 2016-09-13) changed
that, causing a bug on newer kernels.  Fix this by setting the APIC
base in kvm_apic_put.  Another workaround could be to enable
KVM_X2APIC_API_USE_32BIT_IDS, which lets KVM not mangle the APIC ID.

Despite the "don't break userspace" mantra, I believe that this can be
considered a QEMU bug.

I checked kvmtool and it only calls KVM_SET_LAPIC after a KVM_GET_LAPIC,
which avoids the issue in the first place.

Dr. David Alan Gilbert (1):
  kvm: apic: set APIC base as part of kvm_apic_put

Paolo Bonzini (1):
  target-i386: introduce kvm_put_one_msr

 hw/i386/kvm/apic.c     |  2 ++
 target-i386/kvm.c      | 31 ++++++++++++++++++++++---------
 target-i386/kvm_i386.h |  2 ++
 3 files changed, 26 insertions(+), 9 deletions(-)


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