linux-kernel - Re: [PATCH 3/5] x86/ibrs: Add direct access support for MSR_IA32_SPEC

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Date:   Thu, 11 Jan 2018 17:58:11 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Ashok Raj <ashok.raj@...el.com>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Peter Zijlstra <peterz@...radead.org>,
        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>
Subject: Re: [PATCH 3/5] x86/ibrs: Add direct access support for
 MSR_IA32_SPEC_CTRL

On 01/11/2018 05:32 PM, Ashok Raj wrote:
> +static void save_guest_spec_ctrl(struct vcpu_vmx *vmx)
> +{
> +	if (boot_cpu_has(X86_FEATURE_SPEC_CTRL)) {
> +		vmx->spec_ctrl = spec_ctrl_get();
> +		spec_ctrl_restriction_on();
> +	} else
> +		rmb();
> +}

Does this need to be "ifence()"?  Better yet, do we just need to create
a helper for boot_cpu_has(X86_FEATURE_SPEC_CTRL) that does the barrier?