linux-kernel - Re: [RFC 05/10] x86/speculation: Add basic IBRS support infrastructure

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Date:   Wed, 24 Jan 2018 10:34:01 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@....com>,
        Joerg Roedel <joro@...tes.org>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Laura Abbott <labbott@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>, kvm@...r.kernel.org,
        x86@...nel.org
Subject: Re: [RFC 05/10] x86/speculation: Add basic IBRS support
 infrastructure

> > > +	for (i = 0; i < ARRAY_SIZE(spectre_bad_microcodes); i++) {
> > > +		if (c->x86_model == spectre_bad_microcodes[i].model &&
> > > +		    c->x86_mask == spectre_bad_microcodes[i].stepping)
> > > +			return (c->microcode <= spectre_bad_microcodes[i].microcode);
> > > +	}
> > > +	return 0;
> > > +}
> > The above is Intel only, you should check vendor too I think.
> 
> It's in intel.c, called from early_init_intel(). Isn't that sufficient?

Duh, so much for reading skillz on my end ;-)

> > > +		pr_warn("Intel Spectre v2 broken microcode detected; disabling SPEC_CTRL\n");
> > > +		clear_cpu_cap(c, X86_FEATURE_SPEC_CTRL);
> > > +		clear_cpu_cap(c, X86_FEATURE_STIBP);
> > > +		clear_cpu_cap(c, X86_FEATURE_AMD_SPEC_CTRL);
> > > +		clear_cpu_cap(c, X86_FEATURE_AMD_PRED_CMD);
> > > +		clear_cpu_cap(c, X86_FEATURE_AMD_STIBP);
> > > +	}
> > And since its Intel only, what are those AMD features doing there?
> 
> Hypervisors which only want to expose PRED_CMD may do so using the AMD
> feature bit. SPEC_CTRL requires save/restore and live migration
> support, and isn't needed with retpoline anyway (since guests won't be
> calling directly into firmware).

Egads, I suppose that makes some sense, but it does make a horrible
muddle of things.