Enable Hyper-Threading for Linux 5.14

How can I enable Hyper-Threading again for Linux 5.14? I don’t have vulnerable hardware so want maximum performance.

I guess you search this:

On Intel CPUs HT is disabled sometimes cause of security.

I don’t believe that… xD


Did you saw [HowTo] Provide System Information ?
What is pre-history: how did you lost it?
Under which circumstances did you have the HT?

“Sometimes”? May be you have a bit more details on that?

Sorry, but I don’t go into details here. It really depends on the CPU and when it was built… etc.

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Is SMT being disable by default starting with Kernel 5.14?

I can’t seem to find any news of this.

This is the closest I could find, but it doesn’t appear to really affect desktop users:

Yes I have it enabled by default. Great stuff.

cat /sys/devices/system/cpu/smt/active
sudo dmidecode -t processor | grep --color HTT
                HTT (Multi-threading)

Was it not enabled in 5.13 and earlier? On 5.13 I get the same results: SMT/HTT is enabled by default

No idea. The only thing I didn’t touch yet was mitigations, because you have to edit grub and apply.

Honestly, I’d leave that alone. The real-world gains are negligible. SMT/HTT is what makes the biggest difference. Disabling the mitigations against exploits and vulnerabilities barely improves performance, beyond what you gain by enabling SMT/HTT (which is default in recent kernels.) Not worth the risk in my opinion.

  • :turtle: SMT/HTT disabled, mitigations enabled = notable negative performance impact

  • :rabbit2: SMT/HTT enabled, mitigations enabled = near optimal performance

  • :rabbit2: SMT/HTT enabled, mitigations disabled = barely faster than the above combination (not worth the risk)

The question to me is “was HT disabled anyway in any kernel?”. I don’t believe so.

According to some sources, hyper-threading only works on one core, doesn’t matter if you have 4 or 6 cores, but they will still say double threads. So how’s that exactly?

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