Re: [PATCH v2 5/5] x86/idle: Disable IBRS entering mwait idle and enable it on wakeup

[Peter Zijlstra]

On Tue, Jun 20, 2023 at 10:06:25AM -0400, Waiman Long wrote:
> When a user sets "intel_idle.max_cstate=0", it will disable
> intel_idle and fall back to acpi_idle instead. The acpi_idle code
> will then call mwait_idle_with_hints() to enter idle state. So when
> X86_FEATURE_KERNEL_IBRS is enabled, it is necessary to disable IBRS
> within mwait_idle_with_hints() when IRQ was disabled to avoid performance
> degradation on silbing thread running user workload.

Urgh, no, just no. This is nasty.