On Fri, Dec 11, 2020 at 5:39 PM Milan Zamazal <mzamazal@redhat.com> wrote:

TSC frequency is the frequency with which Time Stamp Counter register is
updated, typically a nominal CPU frequency (see
https://en.wikipedia.org/wiki/Time_Stamp_Counter for more details).

You can check the value oVirt gets from libvirt by running

# virsh -r capabilities

and looking at the line like

<counter name='tsc' frequency='2133409000' scaling='no'/>

in the output. Unless frequency scaling is available, the host
frequencies must be almost the same in order to be able to migrate high
performance VMs among them.

Note there is a bug that may cause a migration failure for the VMs even
between hosts with the same frequencies
(https://bugzilla.redhat.com/1821199). But this is apparently not your
case, since the migration is prevented already by Engine.

Regards,
Milan

See here:

[root@ov200 ~]# virsh -r capabilities | grep "name='tsc'"
<counter name='tsc' frequency='3457996000' scaling='no'/>
[root@ov200 ~]#

[root@ov300 ~]# virsh -r capabilities | grep "name='tsc'"
<counter name='tsc' frequency='3457988000' scaling='no'/>
[root@ov300 ~]#

[root@ov301 ~]# virsh -r capabilities | grep "name='tsc'"
<counter name='tsc' frequency='3457997000' scaling='no'/>
[root@ov301 ~]#

The three hosts have the same model cpu

Model name: Intel(R) Xeon(R) CPU X5690 @ 3.47GHz

and slightly different actual frequencies at a certain moment...

But what does it mean so the checkbox

Migrate only to hosts with the same TSC frequency

if even if I don't check it the migration is prevented?

BTW the command lscpu produces exactly the same output on the three hosts, apart "CPU MHz" and corresponding "BogoMIPS" that slightly change each time I run the command.

And the flags for all are:

Flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid dca sse4_1 sse4_2 popcnt aes lahf_lm pti ssbd ibrs ibpb stibp tpr_shadow vnmi flexpriority ept vpid dtherm ida arat flush_l1d

Gianluca