Re: [PATCH 6/7] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC_CTRL feature

[Dave Hansen]

On 01/04/2018 10:33 AM, Borislav Petkov wrote:
>> 2. At run time
>>     echo 0 > /sys/kernel/debug/ibrs_enabled will turn off IBRS
>>     echo 1 > /sys/kernel/debug/ibrs_enabled will turn on IBRS in kernel
>>     echo 2 > /sys/kernel/debug/ibrs_enabled will turn on IBRS in both userspace and kernel
> I am not sure that tristate is really needed. What's wrong with on/off
> only?

Lots of things:

Distros have the tri-state already deployed.
Paranoid people want "IBRS always" aka "ibrs 2".
Future CPUs may have cheap-enough IBRS to not be worth switching it.