Proxmox 6.0 CPU (Host) Flags


Active Member
Nov 13, 2018
I have been using the CPU( Host ) mode with a existing + new flags shown in a new UI.
I have some feedback how can I find out the "default" spec on the UI or the config file. In the UI, default doesn't tell me if it's off or on ( It should just go on off ) .

After some reading, I think the difficulty is that spectre flags in microcode are not known unless explicitly asked to turn on ( I may be wrong after a quick read )

Do enlighten me as the new UI in version 6.0 create more questions from my users rather more intuitive UI.

Tango Charlie


  • Capture.PNG
    46.6 KB · Views: 43
Leaving the setting on a CPU flag on "default" simply means use whatever the CPU model you selected has configured.

In case you select "host", "default" will pass through the option from the host. E.g. if you select "default" on PCID, it will be available to the guest if (and only if) the host has the PCID flag available (check with 'lscpu | grep pcid' on host).

The way that microcode, the CPU (flags), the guest- and host-kernel and a few other parts of the system work together to protect against side-channel attacks is rather complex (read more about Spectre in particular), but what is important in this case, is that the CPU flags just tell the (guest) system which mitigations might be supported, but if they are microcode related, then the host CPU needs to be up-to-date anyway.
  • Like


The Proxmox community has been around for many years and offers help and support for Proxmox VE, Proxmox Backup Server, and Proxmox Mail Gateway.
We think our community is one of the best thanks to people like you!

Get your subscription!

The Proxmox team works very hard to make sure you are running the best software and getting stable updates and security enhancements, as well as quick enterprise support. Tens of thousands of happy customers have a Proxmox subscription. Get yours easily in our online shop.

Buy now!