[PATCH v9 20/21] KVM: x86: Use kernel timekeeping snapshot for monotonic clock

[Sean Christopherson <seanjc@google.com>]

Replace the KVM-private vgettsc()+/do_monotonic() timekeeping
reimplementation with calls to the recently crafted, generic
ktime_get_snapshot_id() interface.  As noted previously, the snapshot
provides both the system time and the raw_cycles (TSC), atomically paired
using a sequence counter.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
[sean: separate from other conversions]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 33 ++++++---------------------------
 1 file changed, 6 insertions(+), 27 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 85c456dd29d5..1513e4ae2353 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1436,29 +1436,6 @@ static inline u64 vgettsc(struct pvclock_clock *clock, u64 *tsc_timestamp,
 	return v * clock->mult;
 }
 
-/*
- * This calculates CLOCK_MONOTONIC at the time of the TSC snapshot, with
- * no boot time offset.
- */
-static int do_monotonic(s64 *t, u64 *tsc_timestamp)
-{
-	struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
-	unsigned long seq;
-	int mode;
-	u64 ns;
-
-	do {
-		seq = read_seqcount_begin(&gtod->seq);
-		ns = gtod->clock.base_cycles;
-		ns += vgettsc(&gtod->clock, tsc_timestamp, &mode);
-		ns >>= gtod->clock.shift;
-		ns += ktime_to_ns(gtod->clock.offset);
-	} while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
-	*t = ns;
-
-	return mode;
-}
-
 static int do_realtime(struct timespec64 *ts, u64 *tsc_timestamp)
 {
 	struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
@@ -1526,12 +1503,14 @@ static bool kvm_get_time_and_clockread(s64 *kernel_ns, u64 *tsc_timestamp)
  */
 bool kvm_get_monotonic_and_clockread(s64 *kernel_ns, u64 *tsc_timestamp)
 {
-	/* checked again under seqlock below */
-	if (!gtod_is_based_on_tsc(pvclock_gtod_data.clock.vclock_mode))
+	struct system_time_snapshot snap = {};
+
+	ktime_get_snapshot_id(CLOCK_MONOTONIC, &snap);
+	if (!kvm_snapshot_has_tsc(&snap, tsc_timestamp))
 		return false;
 
-	return gtod_is_based_on_tsc(do_monotonic(kernel_ns,
-						 tsc_timestamp));
+	*kernel_ns = ktime_to_ns(snap.systime);
+	return true;
 }
 
 /*
-- 
2.55.0.679.g6767b8d81c-goog