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?
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