Re: [tip:x86/pti] x86/speculation: Use IBRS if available before calling into firmware

[Peter Zijlstra]

On Mon, Feb 12, 2018 at 12:50:02PM +0100, Peter Zijlstra wrote:
> On Mon, Feb 12, 2018 at 11:22:11AM +0100, Ingo Molnar wrote:
> > > +static inline void firmware_restrict_branch_speculation_start(void)
> > > +{
> > > +	alternative_msr_write(MSR_IA32_SPEC_CTRL, SPEC_CTRL_IBRS,
> > > +			      X86_FEATURE_USE_IBRS_FW);
> > > +}
> > > +
> > > +static inline void firmware_restrict_branch_speculation_end(void)
> > > +{
> > > +	alternative_msr_write(MSR_IA32_SPEC_CTRL, 0,
> > > +			      X86_FEATURE_USE_IBRS_FW);
> > 
> > BTW., there's a detail that only occurred to me today, this enabling/disabling 
> > sequence is not NMI safe, and it might be called from NMI context:
> 
> Wait, we're doing firmware from NMI? That sounds like a _REALLY_ bad
> idea.

Argh, its that stupid watchdog driver again.. Not only does it call
firmware, it also uses (!raw) spinlock.