linux-kernel - Re: [PATCH] x86: vmx: Allow direct access to MSR_IA32_SPEC

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Message-ID: <1517173013.6624.87.camel@infradead.org>
Date:   Sun, 28 Jan 2018 20:56:53 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        KarimAllah Ahmed <karahmed@...zon.de>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ashok Raj <ashok.raj@...el.com>, daniel.kiper@...cle.com
Subject: Re: [PATCH] x86: vmx: Allow direct access to MSR_IA32_SPEC_CTRL

On Sun, 2018-01-28 at 12:53 -0800, Andy Lutomirski wrote:
> 
> > I believe it does. Guest kernel is protected from any guest userspace
> > predictions learned before IBRS was last set to 1 in *any* mode,
> > including host.
> 
> Hmm, you're probably right.
> 
> I would love to know what awful hack Intel did that resulted in these semantics.

I am not convinced I ever really want to know. I just want it all to go
away in a future CPU with a SPCTR_NO bit in IA32_ARCH_CAPABILITIES.
(Not the IBRS_ALL interim hack).

I think it's a mixture of ongoing checking, and a barrier. And perhaps
varying proportions of each, in different CPU generations. By defining
it thus, they can actually implement it *either* way.
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