[PATCH net] ptp: kvm: report paired cycles for the active clocksource

[Wojtek Wasko <wwasko@nvidia.com>]

The KVM clock-pairing hypercall returns host wall time paired with a
guest TSC value. Cross-timestamping needs that counter expressed in the
guest's active clocksource domain.

ptp_kvm always converts the paired TSC value to kvm-clock units and
reports it as CSID_X86_KVM_CLK, so precise cross-timestamping
(PTP_SYS_OFFSET_PRECISE) fails on guests with TSC clocksource.

Return the paired TSC value directly when TSC cycles are requested and
convert it to kvm-clock units otherwise. If get_device_system_crosststamp()
rejects the counter with -ENODEV and the active clocksource differs from
the requested one, retry once using the active clocksource.

Fixes: a0e136d436de ("PTP: add kvm PTP driver")
Signed-off-by: Wojtek Wasko <wwasko@nvidia.com>
Reviewed-by: Carolina Jubran <cjubran@nvidia.com>
---
 drivers/ptp/ptp_kvm_common.c | 28 +++++++++++++++++++++++++---
 drivers/ptp/ptp_kvm_x86.c    | 13 +++++++++++++
 2 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/drivers/ptp/ptp_kvm_common.c b/drivers/ptp/ptp_kvm_common.c
index 15ccb7dd2ed0..e11fcf961758 100644
--- a/drivers/ptp/ptp_kvm_common.c
+++ b/drivers/ptp/ptp_kvm_common.c
@@ -23,12 +23,13 @@ struct kvm_ptp_clock {
 };
 
 static DEFINE_SPINLOCK(kvm_ptp_lock);
+static enum clocksource_ids kvm_ptp_cs_id = CSID_GENERIC;
 
 static int ptp_kvm_get_time_fn(ktime_t *device_time,
 			       struct system_counterval_t *system_counter,
 			       void *ctx)
 {
-	enum clocksource_ids cs_id;
+	enum clocksource_ids cs_id = *(enum clocksource_ids *)ctx;
 	struct timespec64 tspec;
 	u64 cycle;
 	int ret;
@@ -58,8 +59,29 @@ static int ptp_kvm_get_time_fn(ktime_t *device_time,
 static int ptp_kvm_getcrosststamp(struct ptp_clock_info *ptp,
 				  struct system_device_crosststamp *xtstamp)
 {
-	return get_device_system_crosststamp(ptp_kvm_get_time_fn, NULL,
-					     NULL, xtstamp);
+	enum clocksource_ids cs_id = READ_ONCE(kvm_ptp_cs_id);
+	struct system_time_snapshot snapshot;
+	int ret;
+
+	ret = get_device_system_crosststamp(ptp_kvm_get_time_fn, &cs_id, NULL,
+					    xtstamp);
+	/*
+	 * The paired counter value is not in the units of the current
+	 * clocksource. If the clocksource differs from the one this request
+	 * asked for, ask for the counter in the new units and retry once.
+	 */
+	if (ret == -ENODEV) {
+		ktime_get_snapshot_id(CLOCK_REALTIME, &snapshot);
+		if (snapshot.valid && snapshot.cs_id != cs_id) {
+			cs_id = snapshot.cs_id;
+			WRITE_ONCE(kvm_ptp_cs_id, cs_id);
+			ret = get_device_system_crosststamp(ptp_kvm_get_time_fn,
+							    &cs_id, NULL,
+							    xtstamp);
+		}
+	}
+
+	return ret;
 }
 
 /*
diff --git a/drivers/ptp/ptp_kvm_x86.c b/drivers/ptp/ptp_kvm_x86.c
index 6cea4fe39bcf..4dd55c60d2ce 100644
--- a/drivers/ptp/ptp_kvm_x86.c
+++ b/drivers/ptp/ptp_kvm_x86.c
@@ -92,6 +92,11 @@ int kvm_arch_ptp_get_clock(struct timespec64 *ts)
 	return 0;
 }
 
+/*
+ * *cs_id selects the clocksource @cycle is reported in: CSID_X86_TSC returns
+ * the paired TSC value as is, anything else converts it to kvm-clock units.
+ * On return, *cs_id holds the clocksource actually used.
+ */
 int kvm_arch_ptp_get_crosststamp(u64 *cycle, struct timespec64 *tspec,
 			      enum clocksource_ids *cs_id)
 {
@@ -99,6 +104,14 @@ int kvm_arch_ptp_get_crosststamp(u64 *cycle, struct timespec64 *tspec,
 	unsigned int version;
 	long ret;
 
+	if (*cs_id == CSID_X86_TSC) {
+		ret = kvm_arch_ptp_get_clock(tspec);
+		if (ret)
+			return ret;
+		*cycle = clock_pair->tsc;
+		return 0;
+	}
+
 	src = this_cpu_pvti();
 
 	do {

base-commit: 57889a8f6d49fc153aa0017d13086ab736567f8a
-- 
2.43.0