Re: [PATCH] intel_idle: update spec_ctrl register only when necessary

[Pawan Gupta <pawan.kumar.gupta@linux.intel.com>]

On Tue, Feb 06, 2024 at 03:14:52PM +0800, Li RongQing wrote:
> spec_ctrl maybe 0 when mitigations is off, and writing spec_ctrl MSR
> is expensive operation, so update spec_ctrl only when necessary

Have you observed SPEC_CTRL MSR being written on idle when mitigations
are off?

IIUC, intel_idle_ibrs() will only be called when X86_FEATURE_KERNEL_IBRS
is set:

state_update_enter_method()
{
...
          if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS) &&
                          ((state->flags & CPUIDLE_FLAG_IBRS) || ibrs_off)) {
		  ...
                  state->enter = intel_idle_ibrs;
                  return;
          }

And X86_FEATURE_KERNEL_IBRS wont be set when mitigations are off.