[PATCH v9 21/21] KVM: x86: Use kernel timekeeping snapshot to get walltime+TSC

[Sean Christopherson <seanjc@google.com>]

Replace the KVM-private vgettsc()+/do_do_realtime() 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.

With great pleasure, delete the now unused read_tsc() and vgettsc()

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
[sean: separate from other conversions, express joy at vgettsc()'s demise]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 85 ++++------------------------------------------
 1 file changed, 6 insertions(+), 79 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1513e4ae2353..a638898e528f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1381,82 +1381,6 @@ void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 *user_value)
 
 #ifdef CONFIG_X86_64
 
-static u64 read_tsc(void)
-{
-	u64 ret = (u64)rdtsc_ordered();
-	u64 last = pvclock_gtod_data.clock.cycle_last;
-
-	if (likely(ret >= last))
-		return ret;
-
-	/*
-	 * GCC likes to generate cmov here, but this branch is extremely
-	 * predictable (it's just a function of time and the likely is
-	 * very likely) and there's a data dependence, so force GCC
-	 * to generate a branch instead.  I don't barrier() because
-	 * we don't actually need a barrier, and if this function
-	 * ever gets inlined it will generate worse code.
-	 */
-	asm volatile ("");
-	return last;
-}
-
-static inline u64 vgettsc(struct pvclock_clock *clock, u64 *tsc_timestamp,
-			  int *mode)
-{
-	u64 tsc_pg_val;
-	long v;
-
-	switch (clock->vclock_mode) {
-	case VDSO_CLOCKMODE_HVCLOCK:
-		if (hv_read_tsc_page_tsc(hv_get_tsc_page(),
-					 tsc_timestamp, &tsc_pg_val)) {
-			/* TSC page valid */
-			*mode = VDSO_CLOCKMODE_HVCLOCK;
-			v = (tsc_pg_val - clock->cycle_last) &
-				clock->mask;
-		} else {
-			/* TSC page invalid */
-			*mode = VDSO_CLOCKMODE_NONE;
-		}
-		break;
-	case VDSO_CLOCKMODE_TSC:
-		*mode = VDSO_CLOCKMODE_TSC;
-		*tsc_timestamp = read_tsc();
-		v = (*tsc_timestamp - clock->cycle_last) &
-			clock->mask;
-		break;
-	default:
-		*mode = VDSO_CLOCKMODE_NONE;
-	}
-
-	if (*mode == VDSO_CLOCKMODE_NONE)
-		*tsc_timestamp = v = 0;
-
-	return v * clock->mult;
-}
-
-static int do_realtime(struct timespec64 *ts, 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);
-		ts->tv_sec = gtod->wall_time_sec;
-		ns = gtod->clock.base_cycles;
-		ns += vgettsc(&gtod->clock, tsc_timestamp, &mode);
-		ns >>= gtod->clock.shift;
-	} while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
-
-	ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
-	ts->tv_nsec = ns;
-
-	return mode;
-}
-
 static bool kvm_snapshot_has_tsc(struct system_time_snapshot *snap,
 				 u64 *tsc_timestamp)
 {
@@ -1523,11 +1447,14 @@ bool kvm_get_monotonic_and_clockread(s64 *kernel_ns, u64 *tsc_timestamp)
 static bool kvm_get_walltime_and_clockread(struct timespec64 *ts,
 					   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_REALTIME, &snap);
+	if (!kvm_snapshot_has_tsc(&snap, tsc_timestamp))
 		return false;
 
-	return gtod_is_based_on_tsc(do_realtime(ts, tsc_timestamp));
+	*ts = ktime_to_timespec64(snap.systime);
+	return true;
 }
 #endif
 
-- 
2.55.0.679.g6767b8d81c-goog