This message appears if your ESXi > 6.x host has hyperthreading enabled and is vulnerable to the L1TF CVE’s. Recently I had to rebuild an ESXi 6.0 on a host and got this message after the inital boot when connecting to VCenter:
The message can be surpressed if for example the host has to stay on a specific version of ESXi and you are happy with the security risk of having Hyper Threading enabled. Here is how:
1. Launch the Center Console
2. Browse to Host > Configuration > Advanced Settings.
3. Scroll down to UserVars and click on it.
4. Scroll down to UserVars.SuppressHyperThreadingWarning and enter 1 in the box. The click ok. The warning message will now be surpressed. Note all this does is supress the warning, nothing more.
More info can be found at this VMware KB.