Re: [PATCH] clocksource: don't run watchdog forever

[Feng Tang]

On Thu, Mar 04, 2021 at 03:15:13PM +0100, Thomas Gleixner wrote:
> Feng,
> 
> On Thu, Mar 04 2021 at 15:43, Feng Tang wrote:
> > On Wed, Mar 03, 2021 at 04:50:31PM +0100, Thomas Gleixner wrote:
> >> Anything pre TSC_ADJUST wants the watchdog on. With TSC ADJUST available
> >> we can probably avoid it.
> >> 
> >> There is a caveat though. If the machine never goes idle then TSC adjust
> >> is not able to detect a potential wreckage. OTOH, most of the broken
> >> BIOSes tweak TSC only by a few cycles and that is usually detectable
> >> during boot. So we might be clever about it and schedule a check every
> >> hour when during the first 10 minutes a modification of TSC adjust is
> >> seen on any CPU.
> >
> > I don't have much experience with tsc_adjust, and try to understand it:
> > The 'modification of TSC' here has 2 cases: ? 
> > * First read of 'TSC_ADJUST' MSR of a just boot CPU returns non-zero
> > value
> 
> That's catching stupid BIOSes which set the TSC to random values during
> boot/reboot. That's a one off boot issue and not a real problem. The
> kernel fixes it up and is done with it. Nothing to care about.
> 
> > * Following read of 'TSC_ADJUST' doesn't equal to the 'tsc_adjust' value
> >   saved in per-cpu data.
> 
> That's where we catch broken BIOS/SMI implementations which try to
> "hide" the time wasted in BIOS/SMI by setting the TSC back to the value
> they saved on SMI entry. That was a popular BIOS "feature" some years
> ago, but it seems the BIOS tinkerers finally gave up on it.
 
Thanks for the detailed explaination! I understand now.

> >> Where is this TSC_DISABLE_WRITE bit again?
> 
> I'm serious about this. Once the kernel has taken over a CPU there is
> absolutely no reason for any context to write to the TSC/TSC_ADJUST
> register ever again. So having a mechanism to prevent writes would
> surely help to make the TSC more trustworthy.
> 
> > Also, does the patch ("x86/tsc: mark tsc reliable for qualified platforms")
> > need to wait till this caveat case is solved?
> 
> Yes, but that should be trivial to do. 

Ok, I see.

Thanks,
Feng

> 
> Thanks,
> 
>         tglx