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Date:   Tue, 27 Nov 2018 21:30:29 +0100 (CET)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     "Lendacky, Thomas" <Thomas.Lendacky@amd.com>
cc:     LKML <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
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        Arjan van de Ven <arjan@linux.intel.com>,
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Subject: Re: [patch V2 24/28] x86/speculation: Prepare arch_smt_update() for
 PRCTL mode
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References: <20181125183328.318175777@linutronix.de> <20181125185005.759457117@linutronix.de> <070bd6a5-fd27-2b6d-efea-6d883ba1f8ec@amd.com>
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On Tue, 27 Nov 2018, Lendacky, Thomas wrote:
> On 11/25/2018 12:33 PM, Thomas Gleixner wrote:
> > +/* Update x86_spec_ctrl_base in case SMT state changed. */
> > +static void update_stibp_strict(void)
> >  {
> > -	wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
> > +	u64 mask = x86_spec_ctrl_base & ~SPEC_CTRL_STIBP;
> > +
> > +	if (sched_smt_active())
> > +		mask |= SPEC_CTRL_STIBP;
> > +
> > +	if (mask == x86_spec_ctrl_base)
> > +		return;
> > +
> > +	pr_info("Spectre v2 user space SMT mitigation: STIBP %s\n",
> > +		mask & SPEC_CTRL_STIBP ? "always-on" : "off");
> > +	x86_spec_ctrl_base = mask;
> > +	on_each_cpu(update_stibp_msr, NULL, 1);
> 
> Some more testing using spectre_v2_user=on and I've found that during boot
> up, once the first SMT thread is encountered no more updates to MSRs for
> STIBP are done for any CPUs brought up after that. The first SMT thread
> will cause mask != x86_spec_ctrl_base, but then x86_spec_ctrl_base is set
> to mask and the check always causes a return for subsequent CPUs that are
> brought up.

The above code merily handles the switch between SMT and non-SMT mode,
because there all other online CPUs need to be updated, but after that each
upcoming CPU calls x86_spec_ctrl_setup_ap() which will write the MSR. So
it's all good.

Thanks,

	tglx