linux-kernel - Re: [PATCH v2 2/5] x86/cpufeatures: Add Intel feature bits for Speculation Control

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Message-ID: <1516781583.13558.76.camel@amazon.co.uk>
Date:   Wed, 24 Jan 2018 08:13:03 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     Dave Hansen <dave.hansen@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "arjan@...ux.intel.com" <arjan@...ux.intel.com>,
        "Raslan, KarimAllah" <karahmed@...zon.de>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>
Subject: Re: [PATCH v2 2/5] x86/cpufeatures: Add Intel feature bits for
 Speculation Control

On Tue, 2018-01-23 at 17:28 -0800, Dave Hansen wrote:
> On 01/23/2018 05:23 PM, Woodhouse, David wrote:
> > 
> > On Tue, 2018-01-23 at 10:43 -0800, Dave Hansen wrote:
> ...
> > 
> > > 
> > > > 
> > > >   /* Intel-defined CPU features, CPUID level 0x00000007:0 (EDX), word 18 */
> > > >   #define X86_FEATURE_AVX512_4VNNIW    (18*32+ 2) /* AVX-512 Neural Network Instructions */
> > > >   #define X86_FEATURE_AVX512_4FMAPS    (18*32+ 3) /* AVX-512 Multiply Accumulation Single precision */
> > > > +#define X86_FEATURE_SPEC_CTRL                (18*32+26) /* Speculation Control (IBRS + IBPB) */
> > > > +#define X86_FEATURE_STIBP            (18*32+27) /* Single Thread Indirect Branch Predictors */
> > > > +#define X86_FEATURE_ARCH_CAPABILITIES        (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?  That'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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