Bug#918036: linux: uptime after reboot wrong (kvm-clock related?)

[Ewen McNeill <debbugs@ewen.mcneill.gen.nz>]

I'm also seeing this issue, also on a Debian Linux 4.19 kernel (on 
updated Debian unstable VM), also on KVM, straight after rebooting the 
VM.  But without any suspending involved, I just reboot the VM, and as 
soon as I can log in after rebooting its showing 6+ days of uptime.

The uptime jumps *very* early in the boot sequence, at the point that 
kvm-clock starts reporting, by many days in my case.

Of note, the KVM host here is pretty old (Ubuntu 14.04 LTS, on 
3.13.0-164-generic kernel, and qemu-kvm 2.0.0+dfsg-2ubuntu1.44).  It's 
also naturally a fairly old CPU (Intel Xeon X3450).

So I wonder if part of the trigger is newer (4.19) kernels relying on 
CPU/hypervisor features that older CPUs / older hypervisors do not 
provide/initialise.  And maybe reading an uninitialised value as a result.

From reading the upstream thread, this seems the closest to diagnosis:

https://lore.kernel.org/lkml/20190126020410.GB3796@feynman.vault24.org/

around guest clock sources being chosen, and apparently the upstream 
maintainer has managed to reproduce the issue:

https://lore.kernel.org/lkml/20190126161137.gme4vsrz3xmncbkx@soleen.tm1wkky2jk1uhgkn0ivaxijq1c.bx.internal.cloudapp.net/

by changing the clock source (to HPET), as of late Jan 2019.

Ewen

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ewen@debian-unstable:~$ uptime
 11:21:06 up 6 days, 22:40,  1 user,  load average: 0.88, 0.75, 0.52
ewen@debian-unstable:~$ last | head -5
ewen     pts/0        172.20.254.10    Mon Feb 11 11:10   still logged in
reboot   system boot  4.19.0-2-amd64   Mon Feb 11 11:07   still running
ewen     ttyS0                         Mon Feb 11 11:06 - down   (00:00)
ewen     pts/0        172.20.254.10    Mon Feb 11 11:06 - down   (00:00)
reboot   system boot  4.19.0-2-amd64   Mon Feb 11 11:05 - 11:06  (00:01)
ewen@debian-unstable:~$ date
Mon Feb 11 11:21:21 NZDT 2019
ewen@debian-unstable:~$
ewen@debian-unstable:~$ uname -r
4.19.0-2-amd64
ewen@debian-unstable:~$ dpkg -l | grep linux-image
ii  linux-image-4.19.0-1-amd64           4.19.13-1 
amd64        Linux 4.19 for 64-bit PCs (signed)
ii  linux-image-4.19.0-2-amd64           4.19.16-1 
amd64        Linux 4.19 for 64-bit PCs (signed)
ii  linux-image-amd64                    4.19+102 
amd64        Linux for 64-bit PCs (meta-package)
ewen@debian-unstable:~$
ewen@debian-unstable:~$ sudo dmesg | grep -A 4 "Hypervisor"
[    0.000000] Hypervisor detected: KVM
[    0.000000] kvm-clock: Using msrs 4b564d01 and 4b564d00
[599207.077513] kvm-clock: cpu 0, msr 117d7001, primary cpu clock
[599207.077513] clocksource: kvm-clock: mask: 0xffffffffffffffff 
max_cycles: 0x1cd42e4dffb, max_idle_ns: 881590591483 ns
[599207.077516] tsc: Detected 2659.982 MHz processor
ewen@debian-unstable:~$
ewen@debian-unstable:~$ sudo dmesg | head -20 | grep -v BIOS-e820
[    0.000000] Linux version 4.19.0-2-amd64 
(debian-kernel@lists.debian.org) (gcc version 8.2.0 (Debian 8.2.0-14)) 
#1 SMP Debian 4.19.16-1 (2019-01-17)
[    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-4.19.0-2-amd64 
root=UUID=260df8b9-6a61-4f0e-9625-340dcdaf4017 ro console=ttyS0,9600
[    0.000000] x86/fpu: x87 FPU will use FXSAVE
[    0.000000] BIOS-provided physical RAM map:
[    0.000000] NX (Execute Disable) protection: active
[    0.000000] SMBIOS 2.4 present.
[    0.000000] DMI: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 
01/01/2011
[    0.000000] Hypervisor detected: KVM
[    0.000000] kvm-clock: Using msrs 4b564d01 and 4b564d00
[599207.077513] kvm-clock: cpu 0, msr 117d7001, primary cpu clock
[599207.077513] clocksource: kvm-clock: mask: 0xffffffffffffffff 
max_cycles: 0x1cd42e4dffb, max_idle_ns: 881590591483 ns
[599207.077516] tsc: Detected 2659.982 MHz processor
[599207.078351] e820: update [mem 0x00000000-0x00000fff] usable ==> reserved
ewen@debian-unstable:~$
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ewen@naosdell:~$ head -5 /proc/cpuinfo
processor	: 0
vendor_id	: GenuineIntel
cpu family	: 6
model		: 30
model name	: Intel(R) Xeon(R) CPU           X3450  @ 2.67GHz
ewen@naosdell:~$
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