Re: [Qemu-devel] [PATCH v2 0/2] kvm: fix incorrect LAPIC reset after reboot in x2apic mode

[Dr. David Alan Gilbert]

* Paolo Bonzini (address@hidden) 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 / address@hidden / Manchester, UK