Re: [PATCH linux-next 2/2] x86/xen/time: cleanup xen_tsc_safe_clocksource

[Krister Johansen <kjlx@templeofstupid.com>]

Hi Marcelo,

On Thu, Feb 23, 2023 at 11:34:06AM -0300, Marcelo Tosatti wrote:
> On Mon, Feb 20, 2023 at 08:14:40PM -0800, Krister Johansen wrote:
> > On Mon, Feb 20, 2023 at 11:01:18PM +0100, Thomas Gleixner wrote:
> > > On Mon, Feb 20 2023 at 09:17, Krister Johansen wrote:
> > > > @@ -495,8 +496,7 @@ static int __init xen_tsc_safe_clocksource(void)
> > > >  	/* Leaf 4, sub-leaf 0 (0x40000x03) */
> > > >  	cpuid_count(xen_cpuid_base() + 3, 0, &eax, &ebx, &ecx, &edx);
> > > >  
> > > > -	/* tsc_mode = no_emulate (2) */
> > > > -	if (ebx != 2)
> > > > +	if (ebx != XEN_CPUID_TSC_MODE_NEVER_EMULATE)
> > > >  		return 0;
> > > >  
> > > >  	return 1;
> > > 
> > > What about removing more stupidity from that function?
> > > 
> > > static bool __init xen_tsc_safe_clocksource(void)
> > > {
> > > 	u32 eax, ebx. ecx, edx;
> > >  
> > > 	/* Leaf 4, sub-leaf 0 (0x40000x03) */
> > > 	cpuid_count(xen_cpuid_base() + 3, 0, &eax, &ebx, &ecx, &edx);
> > > 
> > > 	return ebx == XEN_CPUID_TSC_MODE_NEVER_EMULATE;
> > > }
> > 
> > I'm all for simplifying.  I'm happy to clean up that return to be more
> > idiomatic.  I was under the impression, perhaps mistaken, though, that
> > the X86_FEATURE_CONSTANT_TSC, X86_FEATURE_NONSTOP_TSC, and
> > check_tsc_unstable() checks were actually serving a purpose: to ensure
> > that we don't rely on the tsc in environments where it's being emulated
> > and the OS would be better served by using a PV clock.  Specifically,
> > kvmclock_init() makes a very similar set of checks that I also thought
> > were load-bearing.
> 
> kvmclock_init will lower the rating of kvmclock so that TSC clocksource
> can be used instead:
> 
>         /*
>          * X86_FEATURE_NONSTOP_TSC is TSC runs at constant rate
>          * with P/T states and does not stop in deep C-states.
>          *
>          * Invariant TSC exposed by host means kvmclock is not necessary:
>          * can use TSC as clocksource.
>          *
>          */
>         if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
>             boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
>             !check_tsc_unstable())
>                 kvm_clock.rating = 299;

Yes, I saw the change you made to the kvmclock to do this and was
inspired to try to do something similar for Xen:

https://lore.kernel.org/xen-devel/20221216162118.GB2633@templeofstupid.com/

Thanks,

-K