[SRU][Xenial][PATCH v2 2/4] UBUNTU: SAUCE: x86/speculation: Cleanup IBRS runtime control handling

[Juerg Haefliger]

> > +int set_ibrs_enabled(unsigned int val)
> > +{
> > +	int error = 0;
> > +	unsigned int cpu;
> > +	unsigned int prev = ibrs_enabled;
> > +
> > +	mutex_lock(&spec_ctrl_mutex);
> > +
> > +	/* Only enable/disable IBRS if the CPU supports it */
> > +	if (boot_cpu_has(X86_FEATURE_USE_IBRS_FW)) {
> > +		ibrs_enabled = val;
> > +		if (ibrs_enabled != prev)
> > +			pr_info("Spectre V2 : Spectre v2 mitigation: %s "
> > +				"Indirect Branch Restricted Speculation%s\n",
> > +				ibrs_enabled ? "Enabling" : "Disabling",
> > +				ibrs_enabled == 2 ? " (user space)" : "");
> > +
> > +		if (ibrs_enabled == 0) {
> > +			/* Always disable IBRS */
> > +			u64 val = x86_spec_ctrl_base;
> > +
> > +			for_each_online_cpu(cpu)
> > +				wrmsrl_on_cpu(cpu, MSR_IA32_SPEC_CTRL, val);  
> 
> We don't need to handle here the case ibrs_enabled == 1?

No. == 1 means IBRS is temporarily turned on when we enter the kernel and turned
back off again when we leave it, so it's not sticky. Whereas == 0 means always
off (sticky) and == 2 is always on (also sticky).

...Juerg
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