Re: [v6 1/1] x86/bugs: Implement mitigation for Predictive Store Forwarding

[Pawan Gupta]

On 18.05.2021 07:27, Saripalli, RK wrote:
> On 5/17/2021 9:55 PM, Randy Dunlap wrote:
> > Hi again,
> >
> > On 5/17/21 3:00 PM, Ramakrishna Saripalli wrote:
> > > From: Ramakrishna Saripalli <rk.saripalli@xxxxxxx>
> > >
> > > Certain AMD processors feature a new technology called Predictive Store
> > > Forwarding (PSF).
> > >
> > > PSF is a micro-architectural optimization designed to improve the
> > > performance of code execution by predicting dependencies between
> > > loads and stores.
> > >
> > > Incorrect PSF predictions can occur due to two reasons.
> > ...
> >
> > > Kernel parameter predictive_store_fwd_disable has the following values
> > >
> > > - on. Disable PSF on all CPUs.
> > >
> > > - off. Enable PSF on all CPUs.
> > >        This is also the default setting.
> > >
> > > Signed-off-by: Ramakrishna Saripalli<rk.saripalli@xxxxxxx>
> > > ---
> > >  .../admin-guide/kernel-parameters.txt         |  5 +
> > >  arch/x86/include/asm/cpufeatures.h            |  1 +
> > >  arch/x86/include/asm/msr-index.h              |  2 +
> > >  arch/x86/include/asm/nospec-branch.h          |  6 ++
> > >  arch/x86/kernel/cpu/bugs.c                    | 94 +++++++++++++++++++
> > >  5 files changed, 108 insertions(+)
> > >
> > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > > index 04545725f187..a5f694dccb24 100644
> > > --- a/Documentation/admin-guide/kernel-parameters.txt
> > > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > > @@ -3940,6 +3940,11 @@
> > >  			Format: {"off"}
> > >  			Disable Hardware Transactional Memory
> > >
> > > +	predictive_store_fwd_disable=	[X86] This option controls PSF.
> > > +			off - Turns on PSF.
> > > +			on  - Turns off PSF.
> > > +			default : off.
> >
> >
> > and as I did earlier, I still object to "off" meaning PSF is on
> > and "on" meaning that PSF is off.
> >
> > It's not at all user friendly.
> >
> > If it's done this way because that's how the h/w bit is defined/used,
> > that's not a good excuse IMHO.
> >
> > Hm, it sorta seems to be a common "theme" when dealing with mitigations.
> > And too late to fix that.
>
> Based on previous feedback from Thomas Gleixner, I reworded this as a mitigation instead of as a "feature".
> In that vein, all the mitigation code moved into bugs.c like other mitigations, similar to
> spec_control_bypass_disable with an ON/OFF but no prctl/seccomp/auto.

Maybe change the help text to something like:

	on  - Turns on PSF mitigation.
	off - Turns off PSF mitigation.

Thanks,
Pawan