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Date: Fri, 12 Jan 2018 10:51:48 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Dave Hansen <dave.hansen@intel.com>
Cc: Ashok Raj <ashok.raj@intel.com>, linux-kernel@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
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        Dan Williams <dan.j.williams@intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>
Subject: Re: [PATCH 3/5] x86/ibrs: Add direct access support for
 MSR_IA32_SPEC_CTRL
Message-ID: <20180112095148.GP3040@hirez.programming.kicks-ass.net>
References: <1515720739-43819-1-git-send-email-ashok.raj@intel.com>
 <1515720739-43819-4-git-send-email-ashok.raj@intel.com>
 <bb8dcd84-568b-ad74-92c2-646f0d5188a6@intel.com>
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On Thu, Jan 11, 2018 at 05:58:11PM -0800, Dave Hansen wrote:
> 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?

static_cpu_has() + hard asm-goto requirement. Please drop all the above
nonsense on the floor hard.

Let backporters sort out whever they need, don't introduce crap like
that upstream.