pvclock(4): attach even if when PVCLOCK_FLAG_TSC_STABLE is unset

Attaches pvclock with lower priority (500) in case of unstable tsc
(PVCLOCK_FLAG_TSC_STABLE) instead of not attaching at all.  In this state, we do
make sure to return a monotonically increasing number.

This mostly helps openbsd guests on openbsd vmm(4) where a pvclock with unstable
tsc is still better than i8254.

ok mlarkin@

1 files changed, 20 insertions, 12 deletions

diff --git a/sys/dev/pv/pvclock.c b/sys/dev/pv/pvclock.c
index 1325f89887b..6b242f7448d 100644
--- a/sys/dev/pv/pvclock.c
+++ b/sys/dev/pv/pvclock.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: pvclock.c,v 1.4 2019/05/13 15:40:34 pd Exp $	*/
+/*	$OpenBSD: pvclock.c,v 1.5 2019/12/13 06:43:46 pd Exp $	*/
 
 /*
  * Copyright (c) 2018 Reyk Floeter <reyk@openbsd.org>
@@ -29,11 +29,14 @@
 #include <sys/atomic.h>
 
 #include <machine/cpu.h>
+#include <machine/atomic.h>
 #include <uvm/uvm_extern.h>
 
 #include <dev/pv/pvvar.h>
 #include <dev/pv/pvreg.h>
 
+uint pvclock_lastcount;
+
 struct pvclock_softc {
 	struct device		 sc_dev;
 	void			*sc_time;
@@ -141,21 +144,22 @@ pvclock_attach(struct device *parent, struct device *self, void *aux)
 		flags = ti->ti_flags;
 	} while (!pvclock_read_done(ti, version));
 
-	if ((flags & PVCLOCK_FLAG_TSC_STABLE) == 0) {
-		wrmsr(KVM_MSR_SYSTEM_TIME, pa & ~PVCLOCK_SYSTEM_TIME_ENABLE);
-		km_free(sc->sc_time, PAGE_SIZE, &kv_any, &kp_zero);
-		printf(": unstable clock\n");
-		return;
-	}
-
 	sc->sc_tc = &pvclock_timecounter;
 	sc->sc_tc->tc_name = DEVNAME(sc);
 	sc->sc_tc->tc_frequency = 1000000000ULL;
 	sc->sc_tc->tc_priv = sc;
 
+	pvclock_lastcount = 0;
+
 	/* Better than HPET but below TSC */
 	sc->sc_tc->tc_quality = 1500;
 
+	if ((flags & PVCLOCK_FLAG_TSC_STABLE) == 0) {
+		/* if tsc is not stable, set a lower priority */
+		/* Better than i8254 but below HPET */
+		sc->sc_tc->tc_quality = 500;
+	}
+
 	tc_init(sc->sc_tc);
 
 	printf("\n");
@@ -216,10 +220,6 @@ pvclock_get_timecount(struct timecounter *tc)
 		flags = ti->ti_flags;
 	} while (!pvclock_read_done(ti, version));
 
-	/* This bit must be set as we attached based on the stable flag */
-	if ((flags & PVCLOCK_FLAG_TSC_STABLE) == 0)
-		panic("%s: unstable result on stable clock", DEVNAME(sc));
-
 	/*
 	 * The algorithm is described in
 	 * linux/Documentation/virtual/kvm/msr.txt
@@ -231,5 +231,13 @@ pvclock_get_timecount(struct timecounter *tc)
 		delta <<= shift;
 	ctr = ((delta * mul_frac) >> 32) + system_time;
 
+	if ((flags & PVCLOCK_FLAG_TSC_STABLE) != 0)
+		return (ctr);
+
+	if (ctr < pvclock_lastcount)
+		return (pvclock_lastcount);
+
+	atomic_swap_uint(&pvclock_lastcount, ctr);
+
 	return (ctr);
 }