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Subject: Re: [PATCH v2 2/5] x86/cpufeatures: Add Intel feature bits for
 Speculation Control
To:     Dave Hansen <dave.hansen@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "tim.c.chen@linux.intel.com" <tim.c.chen@linux.intel.com>,
        "arjan@linux.intel.com" <arjan@linux.intel.com>,
        "Raslan, KarimAllah" <karahmed@amazon.de>,
        "peterz@infradead.org" <peterz@infradead.org>,
        "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "ak@linux.intel.com" <ak@linux.intel.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>,
        "thomas.lendacky@amd.com" <thomas.lendacky@amd.com>,
        "gregkh@linux-foundation.org" <gregkh@linux-foundation.org>
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Date:   Wed, 24 Jan 2018 08:13:03 +0000
MIME-Version: 1.0
From:   "Woodhouse, David" <dwmw@amazon.co.uk>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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On Tue, 2018-01-23 at 17:28 -0800, Dave Hansen wrote:
> On 01/23/2018 05:23 PM, Woodhouse, David wrote:
> >=20
> > On Tue, 2018-01-23 at 10:43 -0800, Dave Hansen wrote:
> ...
> >=20
> > >=20
> > > >=20
> > > > =C2=A0 /* Intel-defined CPU features, CPUID level 0x00000007:0 (EDX=
), word 18 */
> > > > =C2=A0 #define X86_FEATURE_AVX512_4VNNIW=C2=A0=C2=A0=C2=A0=C2=A0(18=
*32+ 2) /* AVX-512 Neural Network Instructions */
> > > > =C2=A0 #define X86_FEATURE_AVX512_4FMAPS=C2=A0=C2=A0=C2=A0=C2=A0(18=
*32+ 3) /* AVX-512 Multiply Accumulation Single precision */
> > > > +#define X86_FEATURE_SPEC_CTRL=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0(18*32+26) /* S=
peculation Control (IBRS + IBPB) */
> > > > +#define X86_FEATURE_STIBP=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0(18*32+27) /* Single Thread Indirect Branc=
h Predictors */
> > > > +#define X86_FEATURE_ARCH_CAPABILITIES=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0(18*32+29) /* IA32_ARCH_CAPABILITIES MSR (Intel) */
> > > Should we be adding flags (STIBP) for which we currently have no user=
 in
> > > the kernel?
> > They're in an existing word (now) so it costs us absolutely nothing to
> > do so. And they'll be exposed to KVM guests in imminent patches if
> > nothing else.
>
> Doesn't just defining it here generate something in the tables that then
> get exported in /proc/cpuinfo?=C2=A0=C2=A0That's far from our most strict=
 ABI, but
> a single #define here can be seen by users IIRC.

That's true, but still we're *working* on exposing and using these;
let's not go wild adding one feature at a time and having to tweak the
surrounding blacklist/enable/disable/expose logic at every step.
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