linux-kernel - Re: [PATCH v4 4/4] intel_idle: Add ibrs_off module parameter to force disable IBRS

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Date:   Tue, 27 Jun 2023 19:58:15 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Len Brown <lenb@...nel.org>, Jonathan Corbet <corbet@....net>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>
Cc:     linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        x86@...nel.org, linux-pm@...r.kernel.org,
        Robin Jarry <rjarry@...hat.com>, Joe Mario <jmario@...hat.com>
Subject: Re: [PATCH v4 4/4] intel_idle: Add ibrs_off module parameter to force
 disable IBRS

Hi,

On 6/27/23 19:25, Waiman Long wrote:
>  Documentation/admin-guide/pm/intel_idle.rst | 17 ++++++++++++++++-
>  drivers/idle/intel_idle.c                   | 14 ++++++++++++--
>  2 files changed, 28 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/admin-guide/pm/intel_idle.rst b/Documentation/admin-guide/pm/intel_idle.rst
> index b799a43da62e..8604e6d1fe2c 100644
> --- a/Documentation/admin-guide/pm/intel_idle.rst
> +++ b/Documentation/admin-guide/pm/intel_idle.rst

You may take or leave these comments as you choose.

> @@ -216,6 +216,21 @@ are ignored).
>  The idle states disabled this way can be enabled (on a per-CPU basis) from user
>  space via ``sysfs``.
>  
> +The ``ibrs_off`` module parameter is a boolean flag (default to false). It is

                                                       (default false).
or
                                                       (defaults to false).

Then I think it reads better if the next sentence begins with: "If set,"
instead of having it in the middle of the sentence.

> +used to control if IBRS (Indirect Branch Restricted Speculation) should be
> +turned off, if set, when the CPU enters an idle state.  This flag will not
> +affect CPUs that are using Enhanced IBRS which can remain on with little
> +performance impact.
> +
> +For some CPUs, IBRS will be selected as mitigation for Spectre v2 and Retbleed
> +security vulnerabilities by default.  Leaving the IBRS mode on while idling may
> +have a performance impact on its sibling CPU.  The IBRS mode will be turned off
> +by default when the CPU enters into a deep idle state, but not in some
> +shallower ones.  Setting the ``ibrs_off`` module parameter will force the IBRS
> +mode to off when the CPU is in any one of the available idle states.  This may
> +help performance of a sibling CPU at the expense of a slightly higher wakeup
> +latency for the idle CPU.

thanks.
-- 
~Randy