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

Peter Zijlstra
2018-02-12T11:50:58Z

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.