It seems that real mode virtualisation on Nehalem has regressed in 4.2:

On Sun, 2015-10-25 at 10:08 +0100, Stefan Fritsch wrote:
[...]
> I cannot use KVM with 4.2, qemu loops with 100% CPU during seabios
> initialization. Booting with the latest linux-image-4.1.0-2-amd64 fixes
> the issue.
[...]
> kvm tracing shows:
> 
> ...
>  qemu-system-x86-3219  [007] ....  1090.728418: kvm_set_irq: gsi 12 level 0 
> source 0
>  qemu-system-x86-3219  [007] ....  1090.728418: kvm_pic_set_irq: chip 1 pin 4 
> (edge|masked)
>  qemu-system-x86-3219  [007] ....  1090.728419: kvm_ioapic_set_irq: pin 12 
> dst 0 vec=0 (Fixed|physical|edge|masked)
>  qemu-system-x86-3219  [007] ....  1090.728419: kvm_set_irq: gsi 1 level 0 
> source 0
>  qemu-system-x86-3219  [007] ....  1090.728420: kvm_pic_set_irq: chip 0 pin 1 
> (edge|masked)
>  qemu-system-x86-3219  [007] ....  1090.728420: kvm_ioapic_set_irq: pin 1 dst 
> 0 vec=0 (Fixed|physical|edge|masked)
>  qemu-system-x86-3219  [007] ....  1090.728420: kvm_set_irq: gsi 12 level 0 
> source 0
>  qemu-system-x86-3219  [007] ....  1090.728421: kvm_pic_set_irq: chip 1 pin 4 
> (edge|masked)
>  qemu-system-x86-3219  [007] ....  1090.728421: kvm_ioapic_set_irq: pin 12 
> dst 0 vec=0 (Fixed|physical|edge|masked)
>  qemu-system-x86-3219  [007] ....  1090.728723: kvm_set_irq: gsi 0 level 0 
> source 0
>  qemu-system-x86-3219  [007] ....  1090.728724: kvm_pic_set_irq: chip 0 pin 0 
> (edge)
>  qemu-system-x86-3219  [007] ....  1090.728725: kvm_ioapic_set_irq: pin 2 dst 
> 0 vec=0 (Fixed|physical|edge|masked)
>  qemu-system-x86-3219  [007] ....  1090.728725: kvm_set_irq: gsi 0 level 0 
> source 0
>  qemu-system-x86-3219  [007] ....  1090.728725: kvm_pic_set_irq: chip 0 pin 0 
> (edge)
>  qemu-system-x86-3219  [007] ....  1090.728725: kvm_ioapic_set_irq: pin 2 dst 
> 0 vec=0 (Fixed|physical|edge|masked)
>  qemu-system-x86-3219  [007] ....  1090.728726: kvm_set_irq: gsi 0 level 0 
> source 0
>  qemu-system-x86-3219  [007] ....  1090.728726: kvm_pic_set_irq: chip 0 pin 0 
> (edge)
>  qemu-system-x86-3219  [007] ....  1090.728726: kvm_ioapic_set_irq: pin 2 dst 
> 0 vec=0 (Fixed|physical|edge|masked)
>  qemu-system-x86-3221  [000] d...  1090.729926: kvm_write_tsc_offset: vcpu=0 
> prev=18446740943986499809 next=18446740943986499809
>  qemu-system-x86-3221  [000] ....  1090.729927: kvm_track_tsc: vcpu_id 0 
> masterclock 1 offsetmatched 0 nr_online 1 hostclock tsc
>  qemu-system-x86-3221  [000] ....  1090.730004: kvm_update_master_clock: 
> masterclock 1 hostclock tsc offsetmatched 1
>  qemu-system-x86-3221  [000] d...  1090.730010: kvm_entry: vcpu 0
>  qemu-system-x86-3221  [000] ....  1090.730013: kvm_emulate_insn: 
> ffff0000:fff0:ea 5b e0 00 f0 (real)
>  qemu-system-x86-3221  [000] d...  1090.730016: kvm_entry: vcpu 0
>  qemu-system-x86-3221  [000] d...  1090.730017: kvm_exit: reason 
> EPT_VIOLATION rip 0xe05b info 81 0
>  qemu-system-x86-3221  [000] ....  1090.730018: kvm_page_fault: address 
> feffc000 error_code 81
>  qemu-system-x86-3221  [000] d...  1090.730032: kvm_entry: vcpu 0
>  qemu-system-x86-3221  [000] d...  1090.730034: kvm_exit: reason 
> EXCEPTION_NMI rip 0xe05b info 0 80000b08
>  qemu-system-x86-3221  [000] ....  1090.730035: kvm_inj_exception: #DF (0x0)
>  qemu-system-x86-3221  [000] d...  1090.730040: kvm_entry: vcpu 0
>  qemu-system-x86-3221  [000] d...  1090.730041: kvm_exit: reason 
> EXCEPTION_NMI rip 0xfea5 info 0 80000b08
>  qemu-system-x86-3221  [000] ....  1090.730041: kvm_inj_exception: #DF (0x0)
>  qemu-system-x86-3221  [000] d...  1090.730043: kvm_entry: vcpu 0
> ...
[...]
> I am rather surprised that there is no bug report about this yet, so I
> guess it only happens on some hardware. Mine is a Core i7-860.

Ben.

-- 
Ben Hutchings
Beware of bugs in the above code;
I have only proved it correct, not tried it. - Donald Knuth

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