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

[Krister Johansen]

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.

Bah, what I meant to say was emulated, unstable, or otherwise unsuitable
for use as a clocksource.  IOW, even if TSC_MODE_NEVER_EMULATE is
set, it's possible that a user is attempting a migration from a cpu
that's not invariant, and we'd still want to check for that case and
fall back to a PV clocksource, correct?

-K