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Subject: Re: [RFC PATCH 07/13] powerpc/dexcr: Add sysctl entry for SBHE
 system override
From: Benjamin Gray <bgray@linux.ibm.com>
To: Nicholas Piggin <npiggin@gmail.com>, linuxppc-dev@lists.ozlabs.org
Date: Tue, 07 Mar 2023 16:58:22 +1100
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On Tue, 2023-03-07 at 15:30 +1000, Nicholas Piggin wrote:
> On Mon Nov 28, 2022 at 12:44 PM AEST, Benjamin Gray wrote:
> > The DEXCR Speculative Branch Hint Enable (SBHE) aspect controls
> > whether
> > the hints provided by BO field of Branch instructions are obeyed
> > during
> > speculative execution.
> >=20
> > SBHE behaviour per ISA 3.1B:
> >=20
> > 0:=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0The hints provided by BO field of=
 Branch instructions may
> > be
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0ignored during speculat=
ive execution
> >=20
> > 1:=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0The hints provided by BO field of=
 Branch instructions are
> > obeyed
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0during speculative exec=
ution
> >=20
> > Add a sysctl entry to allow changing this aspect globally in the
> > system
> > at runtime:
> >=20
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0/proc/sys/kernel/specul=
ative_branch_hint_enable
> >=20
> > Three values are supported:
> >=20
> > -1:=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0Disable DEXCR SBHE sysctl override
> > =C2=A00:=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0Override and set DEXCR[SBHE] aspe=
ct to 0
> > =C2=A01:=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0Override and set DEXCR[SBHE] aspe=
ct to 1
> >=20
> > Internally, introduces a mechanism to apply arbitrary system wide
> > overrides on top of the prctl() config.
>=20
> Why have an override for this, and not others?
>=20

Should be in the commit message of course, but this aspect bleeds over
to other processes, so a user may wish to prevent all processes from
changing the value. The other aspects are probably only relevant to
their own process, though the implementation here should support
arbitrary system wide overrides.