Re: [patch V2 24/28] x86/speculation: Prepare arch_smt_update() for PRCTL mode

[Thomas Gleixner]

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