linux-kernel - Re: [PATCH v6 2/5] KVM: x86: Add IBPB support

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Date:   Fri, 2 Feb 2018 20:52:58 +0000
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        KarimAllah Ahmed <karahmed@...zon.de>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Ashok Raj <ashok.raj@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v6 2/5] KVM: x86: Add IBPB support

> > No. The AMD feature bits give us more fine-grained support for exposing
> > IBPB or IBRS alone, so we expose those bits on Intel too.  
> 
> But but.. that runs smack against the idea of exposing a platform that
> is as close to emulating the real hardware as possible.

Agreed, and it's asking for problems in the future if for example Intel
or another non AMD vendor did ever use that leaf for something different.

Now whether there ought to be an MSR range every vendor agrees is never
implemented so software can use it is an interesting discussion.

Alan