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.
-K
