Re: [PATCH v3 2/3] intel_idle: Sync up the SPEC_CTRL MSR value to x86_spec_ctrl_current

[Peter Zijlstra]

On Wed, Jun 21, 2023 at 10:46:33PM -0700, Josh Poimboeuf wrote:
> On Wed, Jun 21, 2023 at 08:36:02PM -0400, Waiman Long wrote:
> > When intel_idle_ibrs() is called, it modifies the SPEC_CTRL MSR to
> > 0 in order disable IBRS. However, the new MSR value isn't reflected
> > in x86_spec_ctrl_current which is at odd with the other code that
> > keep track of its state in that percpu variable. Fix that by updating
> > x86_spec_ctrl_current percpu value to always match the content of the
> > SPEC_CTRL MSR.
> 
> Is this fixing an actual bug or is there some other reason for doing
> this?

No actual bug, he did this for his debugfs file -- which is no longer
part of the series. With that on, you can observe the
x86_spec_ctrl_current value while idle.

Arguably that's not even inconsistent because we disable/enable the
thing with IRQs disabled, so nothing can observe the difference.