Public bug reported:

Currently when using linux-kvm (4.4.0-1013 or 4.4.0-1015), if the
virtual machine is configured to copy the host CPU configuration,
`update-grub` freezes and never completes (that's also true when
installing a new kernel).

These are the guest CPU flags (from /proc/cpuinfo) when copying the host
CPU configuration:

flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
mca cmov pat pse36 clflush mmx fxsr sse sse2 ss syscall nx pdpe1gb
rdtscp lm constant_tsc rep_good nopl pni pclmulqdq vmx ssse3 fma cx16
pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes avx f16c
rdrand hypervisor lahf_lm abm invpcid_single tpr_shadow vnmi
flexpriority ept vpid fsgsbase bmi1 avx2 smep bmi2 erms invpcid arat

.. And when using the Hypervisor defaults (that's the scenario where the
issue does not happen):

flags           : fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca
cmov pse36 clflush mmx fxsr sse sse2 syscall nx lm rep_good nopl pni vmx
cx16 x2apic hypervisor lahf_lm tpr_shadow vnmi flexpriority ept vpid

Also, when copying the host CPU configuration, the following stack trace
is shown during boot:

--->8---
Begin: Loading essential drivers ... done.
Begin: Running /scripts/init-premount ... done.
Begin: Mounting root file system ... Begin: Running /scripts/local-top ... done.
Begin: Running /scripts/local-premount ... [    1.899866] raid6: sse2x1   gen() 
 6551 MB/s
[    1.967889] raid6: sse2x1   xor()  5061 MB/s
[    2.035882] raid6: sse2x2   gen()  5855 MB/s
[    2.103866] raid6: sse2x2   xor()  5538 MB/s
[    2.171867] raid6: sse2x4   gen()  9813 MB/s
[    2.239875] raid6: sse2x4   xor()  6767 MB/s
[    2.243885] invalid opcode: 0000 [#1] SMP 
[    2.244520] Modules linked in: raid6_pq(+)
[    2.245129] CPU: 0 PID: 235 Comm: modprobe Not tainted 4.4.0-1015-kvm 
#20-Ubuntu
[    2.245989] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 
Ubuntu-1.8.2-1ubuntu1 04/01/2014
[    2.246881] task: ffff88001f58eb40 ti: ffff88001f5b0000 task.ti: 
ffff88001f5b0000
[    2.247563] RIP: 0010:[<ffffffffc03ab9f1>]  [<ffffffffc03ab9f1>] 
0xffffffffc03ab9f1
[    2.248333] RSP: 0018:ffff88001f5b3bd0  EFLAGS: 00010246
[    2.248865] RAX: 0000000000000001 RBX: ffff88001f5b3c10 RCX: 00000000fffedd22
[    2.249547] RDX: ffff88001f5b3c10 RSI: 0000000000001000 RDI: 0000000000000012
[    2.250222] RBP: 0000000000000012 R08: 000000000000000a R09: 0000000000000124
[    2.250946] R10: ffff88001f695fd0 R11: 0000000000000124 R12: 0000000000001000
[    2.251764] R13: ffff88001f694000 R14: ffff88001f695000 R15: 0000000000000080
[    2.252684] FS:  00007f3335cd0700(0000) GS:ffff88001fc00000(0000) 
knlGS:0000000000000000
[    2.253767] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    2.254558] CR2: 00007ffd707ba398 CR3: 000000001f6a8000 CR4: 0000000000120630
[    2.255548] Stack:
[    2.255852]  ffffffffc03be4a0 0000000000000001 0000000000003624 
0000000000002741
[    2.256997]  ffff88001f694000 ffffffffc03abf38 ffffffffc03c110e 
00000000fffedd22
[    2.258137]  ffffffffc03ae300 ffffffffc03af300 ffffffffc03b0300 
ffffffffc03b1300
[    2.259296] Call Trace:
[    2.259666]  [<ffffffffc03c110e>] ? 0xffffffffc03c110e
[    2.260174]  [<ffffffffc03c1000>] ? 0xffffffffc03c1000
[    2.260681]  [<ffffffff97c00414>] ? 0xffffffff97c00414
[    2.261166]  [<ffffffff97cb2732>] ? 0xffffffff97cb2732
[    2.261671]  [<ffffffff97c92dee>] ? 0xffffffff97c92dee
[    2.262165]  [<ffffffff97c91110>] ? 0xffffffff97c91110
[    2.262672]  [<ffffffff97c913e3>] ? 0xffffffff97c913e3
[    2.263208]  [<ffffffff97c93110>] ? 0xffffffff97c93110
[    2.263709]  [<ffffffff9800e8d5>] ? 0xffffffff9800e8d5
[    2.264237] Code: 98 41 56 48 83 c0 01 41 55 4c 8d 3c c5 00 00 00 00 41 54 
49 89 f4 55 89 fd 53 48 89 d3 4c 8b 2c c2 4e 8b 74 3a 08 e8 4f 2d 87 d7 <c5> fd 
6f 05 27 2c 01 00 c5 e5 ef db 4d 85 e4 0f 84 a7 00 00 00 
[    2.268055] RIP  [<ffffffffc03ab9f1>] 0xffffffffc03ab9f1
[    2.268593]  RSP <ffff88001f5b3bd0>
[    2.268959] ---[ end trace f633f39e1b0e512e ]---
Segmentation fault
done.
Warning: fsck not present, so skipping root file system
--->8---

** Affects: linux-kvm (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1742444

Title:
   Update grub freezes when copying host CPU configuration

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