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

[Dave Hansen]

On 01/04/2018 10:52 AM, Borislav Petkov wrote:
>> Paranoid people want "IBRS always" aka "ibrs 2".
> 
> So why not "IBRS always" or off? No need for the "IBRS only in the
> kernel" setting.

IBRS=1 slows execution down.  If it's on all the time, you pay a
performance cost in userspace.  The assumption is that the user/kernel
boundary switching cost is below the cost of having it on all the time.