* Paolo Bonzini (pbonz...@redhat.com) wrote:
> 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
>
> v1->v2: send the right patches
Yep, that's better - that corresponds to the version you pastebin'd
earlier and it seems to work nicely.
Dave
>
> hw/i386/kvm/apic.c | 2 ++
> target-i386/kvm.c | 31 ++++++++++++++++++++++---------
> target-i386/kvm_i386.h | 2 ++
> 3 files changed, 26 insertions(+), 9 deletions(-)
>
> --
> 2.7.4
>
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
