Re: [patch 4/8] x86/tsc: Verify TSC_ADJUST from idle

[Thomas Gleixner]

On Sun, 20 Nov 2016, Peter Zijlstra wrote:
> On Sat, Nov 19, 2016 at 01:47:37PM -0000, Thomas Gleixner wrote:
> > When entering idle, it's a good oportunity to verify that the TSC_ADJUST
> > MSR has not been tampered with (BIOS hiding SMM cycles). If tampering is
> > detected, emit a warning and restore it to the previous value.
> 
> > +++ b/arch/x86/kernel/process.c
> > @@ -277,6 +277,7 @@ void exit_idle(void)
> >  
> >  void arch_cpu_idle_enter(void)
> >  {
> > +	tsc_verify_tsc_adjust();
> >  	local_touch_nmi();
> >  	enter_idle();
> >  }
> 
> Doing a RDMSR on the idle path isn't going to be popular. That path is
> already way too slow.

Of course we can ratelimit that MSR read with jiffies, but do you have any
better suggestion aside of doing it timer based?

Thanks,

	tglx