[PATCH v4 45/47] x86/kvmclock: Use TSC for sched_clock if it's constant and non-stop

[Sean Christopherson <seanjc@xxxxxxxxxx>]

Prefer the TSC over kvmclock for sched_clock if the TSC is constant,
nonstop, and not marked unstable via command line.  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/Z4hDK27OV7wK572A@xxxxxxxxxx
Reviewed-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 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 1336c24f59cf..cd65ad328637 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -374,7 +374,6 @@ 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;
@@ -394,6 +393,8 @@ 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";
-- 
2.54.0.823.g6e5bcc1fc9-goog