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

[Peter Zijlstra]

On Mon, Feb 12, 2018 at 08:13:31AM -0800, Dave Hansen wrote:
> On 02/12/2018 02:22 AM, Ingo Molnar wrote:
> >> +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:
> 
> FWIW, Tim Chen and I talked about this a bunch.  We ended up just
> saving/restoring the MSR verbatim in the NMI handler the same way we do
> CR3, stashing it in a high general-purpose-register (r%12?).  That costs
> a RDMSR (at least) and an WRMSR (which you can optimize out).  We have a
> patch for that somewhere if anybody wants it.

I would really rather not do that on the NMI path.. And if we _have_ to,
please keep a software shadow of that MSR state, such that we can avoid
touching that MSR 99% of the time.