Re: [PATCH 09/15] x86/split_lock: Explicitly enable or disable #AC for split locked accesses

[Dave Hansen]

On 05/14/2018 11:52 AM, Fenghua Yu wrote:
> By default, we don't set or clear the bit 29 in TEST_CTL MSR 0x33 and
> the bit is inherited from BIOS/hardware setting.
> 
> The kernel parameter "split_lock_ac=on/off" explicitly sets or clears
> the bit during boot time.

The more I think about this...  Why do we need this at boot anyway?
Surely boot-time kernel code can't cause performance issues in the same
way that untrusted repeated userspace can.  Why don't we just let
userspace turn this on?