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

[Waiman Long]

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