[pve-devel] Greatest common cluster cpuflags?

[Mark Casey]

Hello everyone,

I've read that considerable performance improvements can be achieved 
using a non-default kvm CPU with a VM due to the availability of 
additional flags and extension sets. However I understand that this can 
be troublesome for live-migration when cluster Nodes are not on 
homogenous hardware. My own environment specifies a specific CPU for one 
cluster that is homogenous (though I haven't benchmarked it against 
defaults), but I'm stuck with defaults for another cluster which is 
Intel/AMD-mixed and uses CPUs from two different generations.

Would it be possible for Proxmox to track the available cpuflags of 
Nodes and store them in the shared cluster filesystem? I think this 
would allow for running VMs with a greater set of cpuflags than the 
default while reducing the chance of panic (or, whatever) during 
live-migration. It may also be possible to compare the flags that a VM 
was started with and warn if the user attempts to live-migrate it to a 
Node that was added since the VM was last started (a Node which may 
support fewer flags than the other Nodes, causing problems).

I have some interest in playing with this and (I think) a decent idea of 
the scope of work required. Would this idea be considered simply 
impossible, not likely to be adopted/merged under any circumstances, or 
just currently too dangerous without extensive testing? I'm not looking 
for anyone to do my homework for me or write me back a novel...but I 
don't want to waste time if it for sure won't go anywhere.  :D

Thank you,
Mark