Logical cores / SMT with WireGuard

[Tom Li]

On Sun, Feb 17, 2019 at 04:01:36PM +0500, Roman Mamedov wrote:
> On Thu, 14 Feb 2019 18:02:26 +0000
> Lee Yates <rainmakerraw at icloud.com> wrote:
> 
> Sorry, hit "send" before reading the rest of your message.
> 
> > the router runs headless and is awkward to get a monitor to so I can access
> > the BIOS.
> 
> You can toggle it without needing the BIOS.
> It is possible to disable SMT from grub, with Linux kernel boot arguments.
> It even seems possible to disable/enable it without a reboot.
> See https://www.golinuxhub.com/2018/01/how-to-disable-or-enable-hyper.html

Hi all.

The information provided by the original link is out-of-date, it works, but
it's tedious and easy to make a mistake and disable the wrong logical CPU. The
new Linux kernel with L1TF fixes has introduced a SMT kill switch, which is the
standard interface to control SMT.

It's located at

    /sys/devices/system/cpu/smt/control

You can disable SMT by,

    echo "off" > /sys/devices/system/cpu/smt/control

enable it by,

    echo "on" > /sys/devices/system/cpu/smt/control

Or permanently disable it until reboot,

    echo "forceoff" > /sys/devices/system/cpu/smt/control.

More information is available at,

    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/admin-guide/l1tf.rst

I hope it helps.

Cheers,
Tom Li