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Divide error in kvm_unlock_kick()

DISCUSSION Chris Webb 2 participants 9 messages May 28, 2014 – Jun 17, 2014

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Chris Webb

author
#0 lore raw

Running a 3.14.4 x86-64 SMP guest kernel on qemu-2.0, with kvm enabled and -cpu host on a 3.14.4 AMD Opteron host, I'm seeing a reliable kernel panic from the guest shortly after boot. I think is happening in kvm_unlock_kick() in the paravirt_ops code:

divide error: 0000 [#1] PREEMPT SMP

Modules linked in:
CPU: 1 PID: 0 Comm: swapper/1 Not tainted 3.14.4-guest #16
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS Bochs 01/01/2011
task: ffff88007d384880 ti: ffff88007d3b2000 task.ti: ffff88007d3b2000
RIP: 0010:[<ffffffff8102f0cc>]  [<ffffffff8102f0cc>] kvm_unlock_kick+0x63/0x6b
RSP: 0018:ffff88007fc83db0  EFLAGS: 00010046
RAX: 0000000000000005 RBX: 0000000000000000 RCX: 0000000000000003
RDX: 0000000000000003 RSI: ffff88007fd91d40 RDI: 0000000000000008
RBP: ffff88007fd91d40 R08: 0000000000000000 R09: ffffffff8198e840
R10: ffff88007cbc7400 R11: ffff88007cbc9d00 R12: 000000000000cec0
R13: 0000000000000001 R14: ffff88007fd91d40 R15: 0000000000000001
FS:  00007ff42a4d3700(0000) GS:ffff88007fc80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 00007ff42a290006 CR3: 000000007c76d000 CR4: 00000000000406e0
Stack:
 ffff88007fd11d40 ffff88007d361cc0 ffff88007fc8d240 ffffffff81563990
 ffffffff810e42a6 000000038102fa73 0000000000000282 0000000000000000
 ffff88007fd12668 ffff88007fc83ecc 00ffffff00000000 000000000000006b
Call Trace:
 <IRQ> 
 [<ffffffff81563990>] ? _raw_spin_unlock+0x57/0x61
 [<ffffffff810e42a6>] ? load_balance+0x4ff/0x783
 [<ffffffff810e4681>] ? rebalance_domains+0x157/0x20c
 [<ffffffff810e4841>] ? run_rebalance_domains+0x10b/0x148
 [<ffffffff810be7c1>] ? __do_softirq+0xec/0x1fe
 [<ffffffff810beacc>] ? irq_exit+0x48/0x8d
 [<ffffffff815658dd>] ? reschedule_interrupt+0x6d/0x80
 <EOI> 
 [<ffffffff8100a842>] ? hard_enable_TSC+0x2e/0x2e
 [<ffffffff8102fbe1>] ? native_safe_halt+0x2/0x3
 [<ffffffff8100a853>] ? default_idle+0x11/0x14
 [<ffffffff810ed4e7>] ? cpu_startup_entry+0x153/0x1d2
 [<ffffffff810277ad>] ? start_secondary+0x220/0x23c
Code: 0c c5 40 50 87 81 49 8d 04 0c 48 8b 30 48 39 ee 75 ca 8a 40 08 38 d8 75 c3 48 c7 c0 22 b0 00 00 31 db 0f b7 0c 08 b8 05 00 00 00 <0f> 01 c1 5b 5d 41 5c c3 4c 8d 54 24 08 48 83 e4 f0 b9 0a 00 00 
RIP  [<ffffffff8102f0cc>] kvm_unlock_kick+0x63/0x6b
 RSP <ffff88007fc83db0>
---[ end trace 2278d9742b4dff74 ]---
Kernel panic - not syncing: Fatal exception in interrupt

Shutting down cpus with NMI
Kernel Offset: 0x0 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffff9fffffff)

My host kernel config is http://cdw.me.uk/tmp/host-config.txt and the guest config is http://cdw.me.uk/tmp/guest-config.txt with qemu command line:

  qemu-system-x86 -enable-kvm -cpu qemu64 -machine q35 -m 2048 -name $1 \
    -smp sockets=1,cores=4 -pidfile /run/$1.pid -runas nobody \
    -serial stdio -vga none -vnc none -kernel /boot/vmlinuz-guest \
    -append "console=ttyS0 root=/dev/vda" \
    -drive file=/dev/guest/$1,cache=none,format=raw,if=virtio \
    -device virtio-net-pci,netdev=nic,mac=$(< /sys/class/net/$1/address) \
    -netdev tap,id=nic,fd=3 3<>/dev/tap$(< /sys/class/net/$1/ifindex)

I can stop this crash by disabling CONFIG_PARAVIRT_SPINLOCKS in my guest kernel, running with -cpu qemu64 instead of -cpu host, or running with -smp 1 instead of -smp 4. (Removing/changing the -machine q35 makes no difference.)

My CPU flags inside the crashing guest look like this:

fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb lm rep_good nopl extd_apicid pni pclmulqdq ssse3 fma cx16 sse4_1 sse4_2 x2apic popcnt aes xsave avx f16c hypervisor lahf_lm cmp_legacy svm cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw xop fma4 tbm arat npt nrip_save tsc_adjust bmi1

whereas in a (working) -cpu qemu64 guest, they look like this:

fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx lm nopl pni cx16 x2apic popcnt hypervisor lahf_lm cmp_legacy svm abm sse4a

I tried enabling CONFIG_PARAVIRT_DEBUG, but no extra information was reported.

Very happy to do any testing at my end which might help track down what's going on here.

Best wishes,

Chris.

author
#1 lore raw
Quoted 8 lines · Chris Webb

My CPU flags inside the crashing guest look like this:

fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb lm rep_good nopl extd_apicid pni pclmulqdq ssse3 fma cx16 sse4_1 sse4_2 x2apic popcnt aes xsave avx f16c hypervisor lahf_lm cmp_legacy svm cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw xop fma4 tbm arat npt nrip_save tsc_adjust bmi1

Quoted 5 lines

whereas in a (working) -cpu qemu64 guest, they look like this:

fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx lm nopl pni cx16 x2apic popcnt hypervisor lahf_lm cmp_legacy svm abm sse4a

I thought I'd try to bisect on processor flags to see which was/were implicated. The extra flags from -cpu host compared to -cpu qemu64 are:

3dnowprefetch aes arat avx bmi1 cr8_legacy extd_apicid f16c fma fma4 fxsr_opt misalignsse mmxext npt nrip_save osvw pclmulqdq pdpe1gb rep_good sse4_1 sse4_2 ssse3 tbm tsc_adjust vme xop xsave

I can add all of these to -cpu qemu64 with the +FLAG,... syntax and obtain a working guest, but qemu doesn't recognise a handful of them:

CPU feature tsc_adjust not found
CPU feature arat not found
CPU feature cr8_legacy not found
CPU feature extd_apicid not found
CPU feature rep_good not found
CPU feature tsc_adjust not found
Failed to access perfctr msr (MSR c0010001 is ffffffffffffffff) [...]

Doing this results in a working, non-crashing guest, which suggests the behaviour is triggered by one of tsc_adjust, arat, cr8_legacy, extd_apicid or rep_good. However, because qemu doesn't recognise the flags, I can't run with -cpu host,-tsc_adjust,-arat,... to investigate further. :(

Cheers,

Chris.

author
#2 lore raw
In reply to Chris Webb
Quoted 8 lines · Chris Webb

My CPU flags inside the crashing guest look like this:

fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb lm rep_good nopl extd_apicid pni pclmulqdq ssse3 fma cx16 sse4_1 sse4_2 x2apic popcnt aes xsave avx f16c hypervisor lahf_lm cmp_legacy svm cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw xop fma4 tbm arat npt nrip_save tsc_adjust bmi1

Quoted 5 lines

whereas in a (working) -cpu qemu64 guest, they look like this:

fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx lm nopl pni cx16 x2apic popcnt hypervisor lahf_lm cmp_legacy svm abm sse4a

I thought I'd try to bisect on processor flags to see which was/were implicated. The extra flags from -cpu host compared to -cpu qemu64 are:

3dnowprefetch aes arat avx bmi1 cr8_legacy extd_apicid f16c fma fma4 fxsr_opt misalignsse mmxext npt nrip_save osvw pclmulqdq pdpe1gb rep_good sse4_1 sse4_2 ssse3 tbm tsc_adjust vme xop xsave

I can add all of these to -cpu qemu64 with the +FLAG,... syntax and obtain a working guest, but qemu doesn't recognise a handful of them:

CPU feature tsc_adjust not found
CPU feature arat not found
CPU feature cr8_legacy not found
CPU feature extd_apicid not found
CPU feature rep_good not found
CPU feature tsc_adjust not found
Failed to access perfctr msr (MSR c0010001 is ffffffffffffffff) [...]

Doing this results in a working, non-crashing guest, which suggests the behaviour is triggered by one of tsc_adjust, arat, cr8_legacy, extd_apicid or rep_good. However, because qemu doesn't recognise the flags, I can't run with -cpu host,-tsc_adjust,-arat,... to investigate further. :(

Cheers,

Chris.

maintainer
#3 lore raw

Il 29/05/2014 19:45, Chris Webb ha scritto:

Quoted 10 lines
Quoted 8 lines · Chris Webb

My CPU flags inside the crashing guest look like this:

fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb lm rep_good nopl extd_apicid pni pclmulqdq ssse3 fma cx16 sse4_1 sse4_2 x2apic popcnt aes xsave avx f16c hypervisor lahf_lm cmp_legacy svm cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw xop fma4 tbm arat npt nrip_save tsc_adjust bmi1

Quoted 8 lines
Quoted 5 lines

whereas in a (working) -cpu qemu64 guest, they look like this:

fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx lm nopl pni cx16 x2apic popcnt hypervisor lahf_lm cmp_legacy svm abm sse4a

I thought I'd try to bisect on processor flags to see which was/were implicated.

Can you dump the full /proc/cpuinfo?

Paolo

maintainer
#4 lore raw
In reply to Chris Webb

Il 29/05/2014 19:45, Chris Webb ha scritto:

Quoted 10 lines
Quoted 8 lines · Chris Webb

My CPU flags inside the crashing guest look like this:

fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb lm rep_good nopl extd_apicid pni pclmulqdq ssse3 fma cx16 sse4_1 sse4_2 x2apic popcnt aes xsave avx f16c hypervisor lahf_lm cmp_legacy svm cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw xop fma4 tbm arat npt nrip_save tsc_adjust bmi1

Quoted 8 lines
Quoted 5 lines

whereas in a (working) -cpu qemu64 guest, they look like this:

fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx lm nopl pni cx16 x2apic popcnt hypervisor lahf_lm cmp_legacy svm abm sse4a

I thought I'd try to bisect on processor flags to see which was/were implicated.

Can you dump the full /proc/cpuinfo?

Paolo

author
#5 lore raw
Quoted 13 lines · Paolo Bonzini

Il 29/05/2014 19:45, Chris Webb ha scritto:

Quoted 12 lines
Quoted 10 lines · Chris Webb

My CPU flags inside the crashing guest look like this:

fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb lm rep_good nopl extd_apicid pni pclmulqdq ssse3 fma cx16 sse4_1 sse4_2 x2apic popcnt aes xsave avx f16c hypervisor lahf_lm cmp_legacy svm cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw xop fma4 tbm arat npt nrip_save tsc_adjust bmi1

whereas in a (working) -cpu qemu64 guest, they look like this:

Quoted 8 lines
Quoted 6 lines
Quoted 3 lines

fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx lm nopl pni cx16 x2apic popcnt hypervisor lahf_lm cmp_legacy svm abm sse4a

I thought I'd try to bisect on processor flags to see which was/were implicated.

Can you dump the full /proc/cpuinfo?

On the host, it looks like this:

processor : 0
vendor_id : AuthenticAMD
cpu family : 21
model : 2
model name : AMD Opteron(tm) Processor 6328
stepping : 0
microcode : 0x600081c
cpu MHz : 3200.000
cache size : 2048 KB
physical id : 0
siblings : 8
core id : 0
cpu cores : 4
apicid : 32
initial apicid : 0
fpu : yes
fpu_exception : yes
cpuid level : 13
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm constant_tsc rep_good nopl nonstop_tsc extd_apicid amd_dcm aperfmperf pni pclmulqdq monitor ssse3 fma cx16 sse4_1 sse4_2 popcnt aes xsave avx f16c lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ibs xop skinit wdt lwp fma4 tce nodeid_msr tbm topoext perfctr_core perfctr_nb arat cpb hw_pstate npt lbrv svm_lock nrip_save tsc_scale vmcb_clean flushbyasid decodeassists pausefilter pfthreshold bmi1
bogomips : 6399.89
TLB size : 1536 4K pages
clflush size : 64
cache_alignment : 64
address sizes : 48 bits physical, 48 bits virtual
power management: ts ttp tm 100mhzsteps hwpstate cpb eff_freq_ro

[ x8 for processor 0 -> 7; full dump at http://cdw.me.uk/tmp/host-cpuinfo.txt ]

and on the guest it looks like:

processor : 0
vendor_id : AuthenticAMD
cpu family : 21
model : 2
model name : AMD Opteron(tm) Processor 6328
stepping : 0
microcode : 0x1000065
cpu MHz : 3199.946
cache size : 2048 KB
physical id : 0
siblings : 4
core id : 0
cpu cores : 4
apicid : 0
initial apicid : 0
fpu : yes
fpu_exception : yes
cpuid level : 13
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb lm rep_good nopl extd_apicid pni pclmulqdq ssse3 fma cx16 sse4_1 sse4_2 x2apic popcnt aes xsave avx f16c hypervisor lahf_lm cmp_legacy svm cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw xop fma4 tbm arat npt nrip_save tsc_adjust bmi1
bogomips : 6399.89
TLB size : 1536 4K pages
clflush size : 64
cache_alignment : 64
address sizes : 40 bits physical, 48 bits virtual
power management:

[ x4 for processor 0 -> 3; full dump at http://cdw.me.uk/tmp/guest-cpuinfo.txt ]

Many thanks in advance for any pointers.

Best wishes,

Chris.

author
#6 lore raw
In reply to Paolo Bonzini
Quoted 13 lines · Paolo Bonzini

Il 29/05/2014 19:45, Chris Webb ha scritto:

Quoted 12 lines
Quoted 10 lines · Chris Webb

My CPU flags inside the crashing guest look like this:

fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb lm rep_good nopl extd_apicid pni pclmulqdq ssse3 fma cx16 sse4_1 sse4_2 x2apic popcnt aes xsave avx f16c hypervisor lahf_lm cmp_legacy svm cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw xop fma4 tbm arat npt nrip_save tsc_adjust bmi1

whereas in a (working) -cpu qemu64 guest, they look like this:

Quoted 8 lines
Quoted 6 lines
Quoted 3 lines

fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx lm nopl pni cx16 x2apic popcnt hypervisor lahf_lm cmp_legacy svm abm sse4a

I thought I'd try to bisect on processor flags to see which was/were implicated.

Can you dump the full /proc/cpuinfo?

On the host, it looks like this:

processor : 0
vendor_id : AuthenticAMD
cpu family : 21
model : 2
model name : AMD Opteron(tm) Processor 6328
stepping : 0
microcode : 0x600081c
cpu MHz : 3200.000
cache size : 2048 KB
physical id : 0
siblings : 8
core id : 0
cpu cores : 4
apicid : 32
initial apicid : 0
fpu : yes
fpu_exception : yes
cpuid level : 13
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm constant_tsc rep_good nopl nonstop_tsc extd_apicid amd_dcm aperfmperf pni pclmulqdq monitor ssse3 fma cx16 sse4_1 sse4_2 popcnt aes xsave avx f16c lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ibs xop skinit wdt lwp fma4 tce nodeid_msr tbm topoext perfctr_core perfctr_nb arat cpb hw_pstate npt lbrv svm_lock nrip_save tsc_scale vmcb_clean flushbyasid decodeassists pausefilter pfthreshold bmi1
bogomips : 6399.89
TLB size : 1536 4K pages
clflush size : 64
cache_alignment : 64
address sizes : 48 bits physical, 48 bits virtual
power management: ts ttp tm 100mhzsteps hwpstate cpb eff_freq_ro

[ x8 for processor 0 -> 7; full dump at http://cdw.me.uk/tmp/host-cpuinfo.txt ]

and on the guest it looks like:

processor : 0
vendor_id : AuthenticAMD
cpu family : 21
model : 2
model name : AMD Opteron(tm) Processor 6328
stepping : 0
microcode : 0x1000065
cpu MHz : 3199.946
cache size : 2048 KB
physical id : 0
siblings : 4
core id : 0
cpu cores : 4
apicid : 0
initial apicid : 0
fpu : yes
fpu_exception : yes
cpuid level : 13
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb lm rep_good nopl extd_apicid pni pclmulqdq ssse3 fma cx16 sse4_1 sse4_2 x2apic popcnt aes xsave avx f16c hypervisor lahf_lm cmp_legacy svm cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw xop fma4 tbm arat npt nrip_save tsc_adjust bmi1
bogomips : 6399.89
TLB size : 1536 4K pages
clflush size : 64
cache_alignment : 64
address sizes : 40 bits physical, 48 bits virtual
power management:

[ x4 for processor 0 -> 3; full dump at http://cdw.me.uk/tmp/guest-cpuinfo.txt ]

Many thanks in advance for any pointers.

Best wishes,

Chris.

author
#7 lore raw

I realised my original bug report was for a guest kernel compiled without frame pointers which might be unhelpful, so I enabled CONFIG_DEBUG_INFO and CONFIG_FRAME_POINTER, but I don't think this has made the backtrace any more detailed.

Is there anything more I can do to pinpoint what might be going on here?

Cheers,

Chris.

divide error: 0000 [#1] PREEMPT SMP

Modules linked in:
CPU: 1 PID: 1013 Comm: mkdir Not tainted 3.14.4-guest #21
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS Bochs 01/01/2011
task: ffff88007c8cf400 ti: ffff88007c7c6000 task.ti: ffff88007c7c6000
RIP: 0010:[<ffffffff8102ea86>]  [<ffffffff8102ea86>] kvm_unlock_kick+0x69/0x73
RSP: 0000:ffff88007fc83ca8  EFLAGS: 00010046
RAX: 0000000000000005 RBX: 0000000000000000 RCX: 0000000000000002
RDX: 0000000000000002 RSI: ffff88007fd11d40 RDI: ffffffff8198f840
RBP: ffff88007fc83cc0 R08: 0000000000000000 R09: ffffffff8198f840
R10: 000000000000b5e0 R11: 0000000000000005 R12: ffff88007fd11d40
R13: 000000000000cec0 R14: ffff88007d382b80 R15: 0000000000000002
FS:  00007f4c6e265700(0000) GS:ffff88007fc80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f4c6dc9a080 CR3: 000000007c62e000 CR4: 00000000000406e0
Stack:
 0000000000011d40 ffff88007fd11d40 0000000000000002 ffff88007fc83cd0
 ffffffff815852d0 ffff88007fc83d20 ffffffff810dd694 ffff88007fd00000
 0000000000000046 ffff88007d383172 ffff88007d3abe68 0000000000000003
Call Trace:
 <IRQ> 
 [<ffffffff815852d0>] _raw_spin_unlock+0x36/0x5b
 [<ffffffff810dd694>] try_to_wake_up+0x1f4/0x217
 [<ffffffff810dd6f6>] default_wake_function+0xd/0xf
 [<ffffffff810e99f0>] autoremove_wake_function+0xd/0x2f
 [<ffffffff810e944f>] __wake_up_common+0x50/0x7c
 [<ffffffff810e962f>] __wake_up+0x34/0x46
 [<ffffffff810f3b45>] rsp_wakeup+0x1c/0x1e
 [<ffffffff81112e31>] irq_work_run+0x77/0x9b
 [<ffffffff810063e2>] smp_irq_work_interrupt+0x2a/0x31
 [<ffffffff8158739d>] irq_work_interrupt+0x6d/0x80
 [<ffffffff81585336>] ? _raw_spin_unlock_irqrestore+0x41/0x6a
 [<ffffffff810f5402>] rcu_process_callbacks+0x162/0x486
 [<ffffffff810c4140>] ? run_timer_softirq+0x19f/0x1c0
 [<ffffffff810be612>] __do_softirq+0xe1/0x1e9
 [<ffffffff810be8b7>] irq_exit+0x40/0x87
 [<ffffffff810283f1>] smp_apic_timer_interrupt+0x3f/0x4b
 [<ffffffff81586e9d>] apic_timer_interrupt+0x6d/0x80
 <EOI> 
Code: c5 40 50 87 81 49 8d 44 0d 00 48 8b 30 4c 39 e6 75 c9 8a 40 08 38 d8 75 c2 48 c7 c0 22 b0 00 00 31 db 0f b7 0c 08 b8 05 00 00 00 <0f> 01 c1 5b 41 5c 41 5d 5d c3 4c 8d 54 24 08 48 83 e4 f0 b9 0a 
RIP  [<ffffffff8102ea86>] kvm_unlock_kick+0x69/0x73
 RSP <ffff88007fc83ca8>
---[ end trace ed563ea2dedc59b5 ]---
Kernel panic - not syncing: Fatal exception in interrupt

Shutting down cpus with NMI
Kernel Offset: 0x0 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffff9fffffff)

author
#8 lore raw

I see kernel 3.15 is now out, so I retested with 3.15 guest and host. I'm still getting exactly the same guest kernel panic: a divide error in kvm_unlock_kick with -cpu host, but not with -cpu qemu64:

divide error: 0000 [#1] PREEMPT SMP

Modules linked in:
CPU: 1 PID: 781 Comm: mkdir Not tainted 3.15.0-guest #1
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS Bochs 01/01/2011
task: ffff88007cbf6180 ti: ffff880000088000 task.ti: ffff880000088000
RIP: 0010:[<ffffffff8102d1e0>]  [<ffffffff8102d1e0>] kvm_unlock_kick+0x63/0x6b
RSP: 0000:ffff88007fc83d38  EFLAGS: 00010046
RAX: 0000000000000005 RBX: 0000000000000000 RCX: 0000000000000002
RDX: 0000000000000002 RSI: ffff88007fd11d80 RDI: ffffffff81994840
RBP: ffff88007fd11d80 R08: 0000000000000000 R09: ffffffff81994840
R10: ffff88007c480c88 R11: 0000000000000005 R12: 000000000000cec0
R13: ffff88007d38332a R14: 0000000000000002 R15: ffff88007d382d00
FS:  00007fdabf7fd700(0000) GS:ffff88007fc80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fd0643f6509 CR3: 000000007c028000 CR4: 00000000000406e0
Stack:
 0000000000011d80 0000000000000002 ffff88007fd11d80 ffffffff8156f83f
 ffffffff810dba53 0000000000000046 ffff88007fd00000 ffff88007d3bbe70
 ffffffff81845da8 0000000000000003 0000000000000000 0000000000000000
Call Trace:
 <IRQ> 
 [<ffffffff8156f83f>] ? _raw_spin_unlock+0x32/0x55
 [<ffffffff810dba53>] ? try_to_wake_up+0x1ed/0x20f
 [<ffffffff810e78b8>] ? autoremove_wake_function+0x9/0x2a
 [<ffffffff810e739a>] ? __wake_up_common+0x47/0x73
 [<ffffffff810e7547>] ? __wake_up+0x33/0x44
 [<ffffffff8110f10b>] ? irq_work_run+0x72/0x8f
 [<ffffffff81006079>] ? smp_irq_work_interrupt+0x26/0x2b
 [<ffffffff8157185d>] ? irq_work_interrupt+0x6d/0x80
 [<ffffffff810dba64>] ? try_to_wake_up+0x1fe/0x20f
 [<ffffffff8102ad01>] ? native_apic_msr_read+0x6/0x4e
 [<ffffffff8156f89f>] ? _raw_spin_unlock_irqrestore+0x3d/0x65
 [<ffffffff810f2de3>] ? rcu_process_callbacks+0x15e/0x47d
 [<ffffffff810cccf3>] ? execute_in_process_context+0x55/0x55
 [<ffffffff810bdb98>] ? __do_softirq+0xe0/0x1e6
 [<ffffffff810bde23>] ? irq_exit+0x3c/0x81
 [<ffffffff810270e4>] ? smp_apic_timer_interrupt+0x3b/0x46
 [<ffffffff8157135d>] ? apic_timer_interrupt+0x6d/0x80
 <EOI> 
Code: 0c c5 c0 b8 87 81 49 8d 04 0c 48 8b 30 48 39 ee 75 ca 8a 40 08 38 d8 75 c3 48 c7 c0 22 b0 00 00 31 db 0f b7 0c 08 b8 05 00 00 00 <0f> 01 c1 5b 5d 41 5c c3 4c 8d 54 24 08 48 83 e4 f0 b9 0a 00 00 
RIP  [<ffffffff8102d1e0>] kvm_unlock_kick+0x63/0x6b
 RSP <ffff88007fc83d38>
---[ end trace 949b1bf47cc57d09 ]---
Kernel panic - not syncing: Fatal exception in interrupt
Shutting down cpus with NMI
Kernel Offset: 0x0 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffff9fffffff)
---[ end Kernel panic - not syncing: Fatal exception in interrupt

I'm at a complete loss as to what to do next to debug this. Any help would be extremely gratefully received!

I've put 3.15 host and guest configs here:

http://cdw.me.uk/tmp/3.15-guest-config.txt
http://cdw.me.uk/tmp/3.15-host-config.txt

dmesg just after boot here:

http://cdw.me.uk/tmp/3.15-guest-dmesg.txt
http://cdw.me.uk/tmp/3.15-host-dmesg.txt

and /proc/cpuinfo from both host and guest here:

http://cdw.me.uk/tmp/3.15-guest-cpuinfo.txt
http://cdw.me.uk/tmp/3.15-host-cpuinfo.txt

The qemu command line was

  qemu-system-x86 -enable-kvm -cpu host -machine q35 -m 2048 -name omega \
    -smp sockets=1,cores=4 -pidfile /run/omega.pid -runas nobody \
    -serial stdio -vga none -vnc none -kernel /boot/vmlinuz-guest \
    -append "console=ttyS0 root=/dev/vda" \
    -drive file=/dev/guest/omega,cache=none,format=raw,if=virtio \
    -device virtio-rng-pci \
    -device virtio-net-pci,netdev=nic,mac=02:14:72:3c:69:54 \
    -netdev tap,id=nic,fd=3,vhost=on 3<>/dev/tapNNN

but removing the -machine q35 and -device virtio-rng-pci doesn't affect the crash.

Dropping to -smp 1, running with -cpu qemu64, or compiling the guest kernel without paravirtualised spinlock support does remove the panic, albeit at the cost of performance.

Best wishes,

Chris.