linux-kernel - Re: [RFC PATCH 07/13] powerpc/dexcr: Add sysctl entry for SBHE system override

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Message-Id: <CQZWKF2U2GVM.1X6AAD3SM05D6@bobo>
Date:   Tue, 07 Mar 2023 15:30:39 +1000
From:   "Nicholas Piggin" <npiggin@...il.com>
To:     "Benjamin Gray" <bgray@...ux.ibm.com>,
        <linuxppc-dev@...ts.ozlabs.org>
Cc:     <ajd@...ux.ibm.com>, <linux-kernel@...r.kernel.org>,
        <linux-hardening@...r.kernel.org>, <cmr@...escreens.de>
Subject: Re: [RFC PATCH 07/13] powerpc/dexcr: Add sysctl entry for SBHE
 system override

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.
>
> SBHE behaviour per ISA 3.1B:
>
> 0:	The hints provided by BO field of Branch instructions may be
> 	ignored during speculative execution
>
> 1:	The hints provided by BO field of Branch instructions are obeyed
> 	during speculative execution
>
> Add a sysctl entry to allow changing this aspect globally in the system
> at runtime:
>
> 	/proc/sys/kernel/speculative_branch_hint_enable
>
> Three values are supported:
>
> -1:	Disable DEXCR SBHE sysctl override
>  0:	Override and set DEXCR[SBHE] aspect to 0
>  1:	Override and set DEXCR[SBHE] aspect to 1
>
> Internally, introduces a mechanism to apply arbitrary system wide
> overrides on top of the prctl() config.

Why have an override for this, and not others?

Thanks,
Nick