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Message-ID: <9ab9d8ef-36c1-82e9-125f-1f8e6ba5a228@redhat.com>
Date: Wed, 28 Jun 2023 23:03:44 -0400
From: Waiman Long <longman@...hat.com>
To: Randy Dunlap <rdunlap@...radead.org>,
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
On 6/27/23 22:58, Randy Dunlap wrote:
> 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.
Thanks for the suggestion. I will make the change if I have to revise
the patch again for any reason.
Cheers,
Longman
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