xen 4.11.4 incorrect (~3x) cpu frequency reported

[James Dingwall <james-xen@xxxxxxxxxxxxxx>]

Hi,

We have a system which intermittently starts up and reports an incorrect cpu 
frequency:

# grep -i mhz /var/log/kern.log 
Jul 14 17:47:47 dom0 kernel: [    0.000475] tsc: Detected 2194.846 MHz processor
Jul 14 22:03:37 dom0 kernel: [    0.000476] tsc: Detected 2194.878 MHz processor
Jul 14 23:05:13 dom0 kernel: [    0.000478] tsc: Detected 2194.848 MHz processor
Jul 14 23:20:47 dom0 kernel: [    0.000474] tsc: Detected 2194.856 MHz processor
Jul 14 23:57:39 dom0 kernel: [    0.000476] tsc: Detected 2194.906 MHz processor
Jul 15 01:04:09 dom0 kernel: [    0.000476] tsc: Detected 2194.858 MHz processor
Jul 15 01:27:15 dom0 kernel: [    0.000482] tsc: Detected 2194.870 MHz processor
Jul 15 02:00:13 dom0 kernel: [    0.000481] tsc: Detected 2194.924 MHz processor
Jul 15 03:09:23 dom0 kernel: [    0.000475] tsc: Detected 2194.892 MHz processor
Jul 15 03:32:50 dom0 kernel: [    0.000482] tsc: Detected 2194.856 MHz processor
Jul 15 04:05:27 dom0 kernel: [    0.000480] tsc: Detected 2194.886 MHz processor
Jul 15 05:00:38 dom0 kernel: [    0.000473] tsc: Detected 2194.914 MHz processor
Jul 15 05:59:33 dom0 kernel: [    0.000480] tsc: Detected 2194.924 MHz processor
Jul 15 06:22:31 dom0 kernel: [    0.000474] tsc: Detected 2194.910 MHz processor
Jul 15 17:52:57 dom0 kernel: [    0.000474] tsc: Detected 2194.854 MHz processor
Jul 15 18:51:36 dom0 kernel: [    0.000474] tsc: Detected 2194.900 MHz processor
Jul 15 19:07:26 dom0 kernel: [    0.000478] tsc: Detected 2194.902 MHz processor
Jul 15 19:43:56 dom0 kernel: [    0.000154] tsc: Detected 6895.384 MHz processor

The xen 's' debug output:

(XEN) TSC marked as reliable, warp = 0 (count=4)
(XEN) dom1: mode=0,ofs=0x1d1ac8bf8e,khz=6895385,inc=1
(XEN) dom2: mode=0,ofs=0x28bc24c746,khz=6895385,inc=1
(XEN) dom3: mode=0,ofs=0x345696b138,khz=6895385,inc=1
(XEN) dom4: mode=0,ofs=0x34f2635f31,khz=6895385,inc=1
(XEN) dom5: mode=0,ofs=0x3581618a7d,khz=6895385,inc=1
(XEN) dom6: mode=0,ofs=0x3627ca68b2,khz=6895385,inc=1
(XEN) dom7: mode=0,ofs=0x36dd491860,khz=6895385,inc=1
(XEN) dom8: mode=0,ofs=0x377a57ea1a,khz=6895385,inc=1
(XEN) dom9: mode=0,ofs=0x381eb175ce,khz=6895385,inc=1
(XEN) dom10: mode=0,ofs=0x38cab2e260,khz=6895385,inc=1
(XEN) dom11: mode=0,ofs=0x397fc47387,khz=6895385,inc=1
(XEN) dom12: mode=0,ofs=0x3a552762a0,khz=6895385,inc=1

A processor from /proc/cpuinfo in dom0:

processor       : 3
vendor_id       : GenuineIntel
cpu family      : 6
model           : 85
model name      : Intel(R) Xeon(R) D-2123IT CPU @ 2.20GHz
stepping        : 4
microcode       : 0x2000065
cpu MHz         : 6895.384
cache size      : 8448 KB
physical id     : 0
siblings        : 4
core id         : 0
cpu cores       : 1
apicid          : 0
initial apicid  : 0
fpu             : yes
fpu_exception   : yes
cpuid level     : 13
wp              : yes
flags           : fpu de tsc msr pae mce cx8 apic sep mca cmov pat clflush acpi 
mmx fxsr sse sse2 ss ht syscall nx lm constant_tsc rep_good nopl nonstop_tsc 
cpuid pni pclmulqdq monitor est ssse3 fma cx16 sse4_1 sse4_2 movbe popcnt aes 
xsave avx f16c rdrand hypervisor lahf_lm abm 3dnowprefetch intel_ppin ssbd ibrs 
ibpb stibp fsgsbase bmi1 hle avx2 bmi2 erms rtm avx512f avx512dq rdseed adx 
clflushopt clwb avx512cd avx512bw avx512vl xsaveopt xsavec xgetbv1 md_clear
bugs            : null_seg cpu_meltdown spectre_v1 spectre_v2 spec_store_bypass 
l1tf mds swapgs taa itlb_multihit
bogomips        : 13790.76
clflush size    : 64
cache_alignment : 64
address sizes   : 46 bits physical, 48 bits virtual
power management:

Xen has been built at 310ab79875cb705cc2c7daddff412b5a4899f8c9 from the 
stable-4.12 branch.  The system is a supermicro server, model X11SDV-4C-TP8F.  
I'm not sure if the incorrect value has been read from hardware or Xen has 
miscalculated the frequency so any pointers on things to examine would be 
welcome.

Thanks,
James