Prefer the TSC over kvmclock for sched_clock if the TSC is constant and
nonstop. I.e. use the same criteria as tweaking the clocksource rating so
that TSC is preferred over kvmclock. Per the below comment from
native_sched_clock(), sched_clock is more tolerant of slop than
clocksource; using TSC for clocksource but not sched_clock makes little to
no sense, especially now that KVM CoCo guests with a trusted TSC use TSC,
not kvmclock.
/*
* Fall back to jiffies if there's no TSC available:
* ( But note that we still use it if the TSC is marked
* unstable. We do this because unlike Time Of Day,
* the scheduler clock tolerates small errors and it's
* very important for it to be as fast as the platform
* can achieve it. )
*/
The only advantage of using kvmclock is that doing so allows for early
and common detection of PVCLOCK_GUEST_STOPPED, but that code has been
broken for over two years with nary a complaint, i.e. it can't be
_that_ valuable. And as above, certain types of KVM guests are losing
the functionality regardless, i.e. acknowledging PVCLOCK_GUEST_STOPPED
needs to be decoupled from sched_clock() no matter what.
Link: https://lore.kernel.org/all/[email protected]
Reviewed-by: David Woodhouse <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
---
arch/x86/kernel/kvmclock.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 22e8855fcd4d..bc98ebb8587d 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -396,7 +396,6 @@ void __init kvmclock_init(bool prefer_tsc)
PVCLOCK_TSC_STABLE_BIT;
}
- kvm_sched_clock_init(stable);
if (!x86_init.hyper.get_tsc_khz)
x86_init.hyper.get_tsc_khz = kvmclock_get_tsc_khz;
@@ -416,6 +415,8 @@ void __init kvmclock_init(bool prefer_tsc)
*/
if (prefer_tsc)
kvm_clock.rating = 299;
+ else
+ kvm_sched_clock_init(stable);
clocksource_register_hz(&kvm_clock, NSEC_PER_SEC);
pv_info.name = "KVM";
--
2.55.0.rc0.799.gd6f94ed593-goog
