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

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Message-ID: <1517171968.6624.80.camel@infradead.org>
Date:   Sun, 28 Jan 2018 20:39:28 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        KarimAllah Ahmed <karahmed@...zon.de>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     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 15:21 -0500, Konrad Rzeszutek Wilk wrote:
> >To avoid the overhead of atomically saving and restoring the MSR_IA32_SPEC_CTRL
> >for guests that do not actually use the MSR, only add_atomic_switch_msr when a
> >non-zero is written to it.
> 
> 
> We tried this and found that it was about 3% slower that doing the
> old way of rdmsr and wrmsr.
> 
> But that was also with the host doing IBRS  as well.

The common case will be that neither host nor guest are doing IBRS.
Until the guest touches the MSR we do absolutely *nothing* with it,
which is definitely the fastest option.
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