[Bug 1524069] Re: [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes

[Mathew Hodson]

** No longer affects: kvm (Ubuntu)

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Title:
  [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in
  raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial
  KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes kernel
  exception]

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[Bug 1524069] Re: [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes

[William Grant]

One could argue that libvirt should exclude x2apic from the host-model
checks, as it's emulated by qemu whether or not the host supports it.

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Title:
  [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in
  raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial
  KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes kernel
  exception]

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[Bug 1524069] Re: [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes

[William Grant]

If you need avx2 support, --cpu Haswell-noTSX,-x2apic works on Haswell
desktop/laptop chips.

The most confusing problem is that qemu's definition of "Haswell" is
actually Haswell-E, -EP and -EX; Haswell itself lacks x2apic, which
qemu's Haswell requires. x2apic dates back to Nehalem, but qemu's CPU
definitions only include it back to Sandy Bridge, so a standard desktop
or laptop Haswell CPU falls all the back to Westmere and then adds flags
including avx, avx2 and xsave on top of that[0].

Advertising support for AVX and AVX2 is just a matter of setting
CPUID.1:ECX.AVX and CPUID.7:EBX.AVX2, but the instructions won't
actually work unless XCR0.AVX is set, and kvm_load_guest_xcr0 only sets
XCR0 from the guest if the guest's XR4.OSXSAVE is set. The guest's
fpu__init_cpu_xstate only sets CR4.OSXSAVE when xfeatures_mask is non-
zero, and xfeatures_mask is calculated by fpu__init_system_xstate from
CPUID.(EAX=0DH,ECX=0), which doesn't exist on qemu's Westmere
(level=0xb), so XCR0.AVX remains unset and AVX2 instructions #UD.

The bug is probably that raid6_have_avx2 only checks that AVX2 is
supported in CPUID, not that it's enabled. Checking for
X86_FEATURE_OSXSAVE might work, though I'm not sure if the value checked
by boot_cpu_has is stored too early for that.

I am also a little suspicious of kvm_load_guest_xcr0's CR4.OSXSAVE
guard. The Intel manuals state that XSAVE, XSRSTOR, XGETBV and XSETBV
require the flag to be set, but KVM won't restore a guest's existing
XCR0 unless OSXSAVE is still set.


[0] "-cpu 
Westmere,+invpcid,+erms,+bmi2,+smep,+avx2,+bmi1,+fsgsbase,+abm,+rdtscp,+pdpe1gb,+rdrand,+f16c,+avx,+osxsave,+xsave,+tsc-deadline,+movbe,+pcid,+pdcm,+xtpr,+fma,+tm2,+est,+vmx,+ds_cpl,+monitor,+dtes64,+pclmuldq,+pbe,+tm,+ht,+ss,+acpi,+ds,+vme"

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Title:
  [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in
  raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial
  KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes kernel
  exception]

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[Bug 1524069] Re: [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes

[Serge Victor]

The bug persists in kernel 4.3.0-2-generic #11-Ubuntu.

@Mike - thanks for a workaround, it works, indeed :-)

--cpu=host-model-only

for virt-manager.

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Title:
  [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in
  raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial
  KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes kernel
  exception]

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[Bug 1524069] Re: [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes

[Serge Victor]

To be honest, it's pretty annoying bug, as I am not able to install any
xenial VM :-(

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Title:
  [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in
  raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial
  KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes kernel
  exception]

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[Bug 1524069] Re: [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes

[Mike Pontillo]

@Serge, A workaround for me is to use "hypervisor default" in  virt-
manager. I'm not sure what the equivalent is in virt-install, but maybe
using --cpu=host-model-only would be a workaround?

>From the man page:

   Expose the host CPUs configuration to the guest. This enables 
the guest to take advantage of many
   of the host CPUs features (better performance), but may cause 
issues if migrating the guest to a
   host without an identical CPU.

   --cpu host-model-only
   Expose the nearest host CPU model configuration to the guest.  
It is the best CPU which can be
   used for a guest on any of the hosts.

   Use --cpu=? to see a list of all available sub options. Complete 
details at
   

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Title:
  [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in
  raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial
  KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes kernel
  exception]

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[Bug 1524069] Re: [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes

[Andy Whitcroft]

** Summary changed:

- [trusty] 3.13.0-68 raid6-pq panic in raid6_avx21_gen_syndrome() while probing 
grub devices (was: Xenial KVM: updating Trusty guest from 3.13.0-68 to 
3.13.0-71 causes kernel exception)
+ [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in 
raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial KVM: 
updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes kernel exception]

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Title:
  [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in
  raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial
  KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes kernel
  exception]

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[Bug 1524069] Re: [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes

[Chris J Arges]

Ok I attempted to reproduce without success. What I tried:
- Xenial on Xenial on machine/guest with avx2
- Trusty 3.13.0-{68,71} on Xenial on machine/guest with avx2

In both of these instances I modprobed raid6_pq, in the trusty instance
I triggered an upgrade from 68 to 71 and neither of these caused a
segfault.

The following information would be useful in debugging this:

1) The differences between the guests /proc/cpuinfo with it reproducing and 
not-reproducing. This can verify the avx2 bit is to blame.
2) Can you simply 'sudo modprobe -r raid6_pq && sudo modprobe raid6_pq' to 
trigger this issue?
3) Can you test with a xenial guest and a similar configuration to see if this 
still triggers the issue?
4) If you have the ability testing with different versions of hypervisor 
(trusty vs xenial) might also be useful.

Thanks,
--chris j arges

** Changed in: linux (Ubuntu)
 Assignee: (unassigned) => Chris J Arges (arges)

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Title:
  [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in
  raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial
  KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes kernel
  exception]

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[Bug 1524069] Re: [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes

[Mike Pontillo]

On my Trusty VM with "[x] Copy host CPU configuration" checked in virt-
manager, running 'sudo modprobe -r raid6_pq && sudo modprobe raid6_pq'
is enough to reproduce the kernel exception in dmesg (though I didn't
see it print "Segmentation Fault", so that may be somewhat of a red
herring).

Here's /proc/cpuinfo on the guest:
http://paste.ubuntu.com/13868715/

And here's /proc/cpuinfo on the hypervisor host:
http://paste.ubuntu.com/13868744/

I have a separate issue (which I have not filed, because I haven't
triaged, and am not sure I can reproduce yet) that was blocking me from
creating a Xenial guest, but I can try again shortly.

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Title:
  [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in
  raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial
  KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes kernel
  exception]

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[Bug 1524069] Re: [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes

[Launchpad Bug Tracker]

Status changed to 'Confirmed' because the bug affects multiple users.

** Changed in: kvm (Ubuntu)
   Status: New => Confirmed

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Title:
  [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in
  raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial
  KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes kernel
  exception]

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[Bug 1524069] Re: [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes

[Serge Victor]

It affects me, trying to bootstrap xenial vm on xenial, getting OOPS
like above.

# virt-install --name $SHORT \
--ram $MEM \
--vcpus $CPUS \

--location=http://hk.archive.ubuntu.com/ubuntu/dists/xenial/main/installer-amd64/
 \
--os-type=linux --os-variant=virtio26 \
--bridge=siec,model=virtio,mac=$MAC \
--disk path=$DISK \
--cpuset=0,1,2 --cpu=host \
--autostart --nographics \
--extra-args="auto=true priority=critical locale=pl_PL keymap=pl 
debian-installer/allow_unauthenticated_ssl=true console-setup/ask_detect=false 
netcfg/hostname=$HOST url=https://domain.com/ubuntu.pre4seed.cfg console=tty0 
console=ttyS0,115200n8 serial"

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Title:
  [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in
  raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial
  KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes kernel
  exception]

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