RE: [PATCH v6] x86: intel_epb: Add earlyparam option to keep bias at performance

["Durrant, Paul" <pdurrant@amazon.co.uk>]

> -----Original Message-----
> From: Jack Allister <jalliste@amazon.com>
> Sent: 04 January 2024 09:06
> Cc: Allister, Jack <jalliste@amazon.co.uk>; Rafael J . Wysocki
> <rafael@kernel.org>; Durrant, Paul <pdurrant@amazon.co.uk>; Wang, Jue
> <juew@amazon.com>; Usama Arif <usama.arif@bytedance.com>; Jonathan Corbet
> <corbet@lwn.net>; Thomas Gleixner <tglx@linutronix.de>; Ingo Molnar
> <mingo@redhat.com>; Borislav Petkov <bp@alien8.de>; Dave Hansen
> <dave.hansen@linux.intel.com>; x86@kernel.org; H. Peter Anvin
> <hpa@zytor.com>; Paul E. McKenney <paulmck@kernel.org>; Randy Dunlap
> <rdunlap@infradead.org>; Tejun Heo <tj@kernel.org>; Peter Zijlstra
> <peterz@infradead.org>; Yan-Jie Wang <yanjiewtw@gmail.com>; Hans de Goede
> <hdegoede@redhat.com>; linux-doc@vger.kernel.org; linux-
> kernel@vger.kernel.org
> Subject: [PATCH v6] x86: intel_epb: Add earlyparam option to keep bias at
> performance
> 
> Buggy BIOSes may not set a sane boot-time Energy Performance Bias (EPB).
> A result of this may be overheating or excess power usage. The kernel
> overrides any boot-time EPB "performance" bias to "normal" to avoid this.
> 
> When used in data centers it is preferable keep the EPB at "performance"
> when performing a live-update of the host kernel via a kexec to the new
> kernel. This is due to boot-time being critical when performing the kexec
> as running guest VMs will perceieve this as latency or downtime.
> 
> On Intel Xeon Ice Lake platforms it has been observed that a combination
> of
> EPB being set to "normal" alongside HWP (Intel Hardware P-states) being
> enabled/configured during or close to the kexec causes an increases the
> live-update/kexec downtime by 7 times compared to when the EPB is set to
> "performance".
> 
> Introduce a command-line parameter, "intel_epb=preserve", to skip the
> "performance" -> "normal" override/workaround. This maintains prior
> functionality when no parameter is set, but adds in the ability to stay at
> performance for a speedy kexec if a user wishes.
> 
> Signed-off-by: Jack Allister <jalliste@amazon.com>
> Acked-by: Rafael J. Wysocki <rafael@kernel.org>
> Cc: Paul Durrant <pdurrant@amazon.com>
> Cc: Jue Wang <juew@amazon.com>
> Cc: Usama Arif <usama.arif@bytedance.com>
> ---
>  .../admin-guide/kernel-parameters.txt         |  9 ++++++++
>  arch/x86/kernel/cpu/intel_epb.c               | 22 +++++++++++++++++--
>  2 files changed, 29 insertions(+), 2 deletions(-)
> 

Reviewed-by: Paul Durrant <pdurrant@amazon.com>