linux-kernel - Re: [PATCH 29/35] x86/speculation: Add IPBP support

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Message-ID: <20180118184625.kqwkrpxutiolvqyb@pd.tnic>
Date:   Thu, 18 Jan 2018 19:46:25 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Jason Baron <jbaron@...mai.com>
Subject: Re: [PATCH 29/35] x86/speculation: Add IPBP support

On Thu, Jan 18, 2018 at 12:35:23PM -0600, Josh Poimboeuf wrote:
> Maybe I missed the memo, why do we need both X86_FEATURE_IBPB and
> X86_FEATURE_AMD_IBPB?

So AMD_IBPB is a different CPUID bit in a different CPUID function
and on Intel, IBPB is set only when X86_FEATURE_SPEC_CTRL - see
specctrl_init_ibpb() in that same patch.

IOW, CPUID(7).EDAC[26] means both IBRS and IBPB on Intel.

So, in the end of the day, X86_FEATURE_IBPB is the feature bit we'll be
testing and X86_FEATURE_AMD_IBPB is used only to set X86_FEATURE_IBPB.

:-)

More confused?

-- 
Regards/Gruss,
    Boris.

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