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Message-ID: <1517049146.6624.5.camel@infradead.org>
Subject: Re: [PATCH] x86/cpufeatures: Cleanup AMD speculation feature bits
From:   David Woodhouse <dwmw2@infradead.org>
To:     Borislav Petkov <bp@alien8.de>
Cc:     Tom Lendacky <thomas.lendacky@amd.com>, x86-ml <x86@kernel.org>,
        linux-tip-commits@vger.kernel.org, hpa@zytor.com,
        gregkh@linuxfoundation.org, tglx@linutronix.de,
        linux-kernel@vger.kernel.org, mingo@kernel.org
In-Reply-To: <20180127093722.t6dn4sgocihjjq37@pd.tnic>
References: <tip-5d10cbc91d9eb5537998b65608441b592eec65e7@git.kernel.org>
         <20180126184139.rcfbtk7dvj7kmyfn@pd.tnic>
         <1516992318.30244.271.camel@infradead.org>
         <20180126184915.ioqewp56orj2qhrt@pd.tnic>
         <7094ed9b-40f7-ba2b-55a6-cc5ab0b06bb9@amd.com>
         <20180126215209.vqdxh5p672tcdst6@pd.tnic>
         <1517003984.30244.299.camel@infradead.org>
         <20180126221026.hc2hk23zsqbqhkif@pd.tnic>
         <2cea933f-7e06-26d5-95cf-41df8308a0f8@amd.com>
         <1517045268.30244.305.camel@infradead.org>
         <20180127093722.t6dn4sgocihjjq37@pd.tnic>
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Date:   Sat, 27 Jan 2018 10:32:26 +0000
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On Sat, 2018-01-27 at 10:37 +0100, Borislav Petkov wrote:
> On Sat, Jan 27, 2018 at 09:27:48AM +0000, David Woodhouse wrote:
> >=20
> > http://david.woodhou.se/cleanup-feature-bits.patch=C2=A0on top of my fu=
ll
> > tree?
> @@ -223,7 +223,7 @@ static inline void indirect_branch_prediction_barrier=
(void)
> =C2=A0				=C2=A0"movl %[val], %%eax\n\t"
> =C2=A0				=C2=A0"movl $0, %%edx\n\t"
> =C2=A0				=C2=A0"wrmsr",
> -				=C2=A0X86_FEATURE_IBPB)
> +				=C2=A0X86_FEATURE_USE_IBPB)
>=20
> I still don't think that's the right approach: I'd call the
> software-defined, synthetic features
>=20
> X86_FEATURE_IBPB
> X86_FEATURE_IBRS
> X86_FEATURE_STIBP
>=20
> then make *them* visible in /proc/cpuinfo and use them everywhere in the
> code.

No because cpuinfo should be information about the CPU. For details of
what mitigations are *actually* in use on this kernel, you want
/sys/=E2=80=A6/cpu/vulnerabilities, which might not even be readable by a n=
on-
root user.

That's why I've used the names that we want to see in cpuinfo, for the
basic CPU functionality.

> Only the vendor-specific detection code will set the synthetic ones when
> it detects a corresponding vendor-specific one.
>=20
> This way one *only* concentrates on the three above everywhere and
> only low-level, early, vendor-specific code takes care to set the
> corresponding synthetic features based on the actual hardware bits it
> detects.
>=20
> I think that unifies the view both to the user *and* to the rest of the
> kernel which should not care about the actual name of a hardware feature
> bit.
>=20
> And then you avoid coders scratching heads, asking, so what should I
> use, X86_FEATURE_IBPB or X86_FEATURE_USE_IBPB.

Does it exist, vs. whether the kernel is *using* it. The latter being a
little bit of a hack because alternatives *only* let us do this stuff
based on "CPU features", which is why X86_FEATURE_PTI exists.

That one probably shouldn't be user-visible in /proc/cpuinfo *either*,
should it?

> Instead you call IBPB the synthetic one and the hardware feature name is
> something different like PRED_CMD or so. This will drop the confusion
> additionally.

I think I covered this, but for clarity: No, because the *hardware*
feature is the one we want to be called just "ibpb" in /proc/cpuinfo.

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