[patch 1/2] KVM: x86: switch KVMCLOCK base to CLOCK_MONOTONIC_RAW

[Marcelo Tosatti <mtosatti@xxxxxxxxxx>]

Commit 0bc48bea36d1 ("KVM: x86: update master clock before computing kvmclock_offset")
switches the order of operations to avoid the conversion 

TSC (without frequency correction) ->
system_timestamp (with frequency correction), 

which might cause a time jump.

However, it leaves any other masterclock update unsafe, which includes, 
at the moment:

        * HV_X64_MSR_REFERENCE_TSC MSR write.
        * TSC writes.
        * Host suspend/resume.

Avoid the time jump issue by using frequency uncorrected
CLOCK_MONOTONIC_RAW clock. 

Its the guests time keeping software responsability
to track and correct a reference clock such as UTC.

Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>

---
 arch/x86/kvm/x86.c |   68 +++++++++++++++++++++++++++++++++++------------------
 1 file changed, 46 insertions(+), 22 deletions(-)

Index: linux-2.6.git/arch/x86/kvm/x86.c
===================================================================
--- linux-2.6.git.orig/arch/x86/kvm/x86.c	2018-08-09 10:32:51.151942374 -0300
+++ linux-2.6.git/arch/x86/kvm/x86.c	2018-08-09 11:49:47.111293686 -0300
@@ -1240,20 +1240,25 @@
 }
 
 #ifdef CONFIG_X86_64
+struct pvclock_clock {
+	int vclock_mode;
+	u64 cycle_last;
+	u64 mask;
+	u32 mult;
+	u32 shift;
+};
+
 struct pvclock_gtod_data {
 	seqcount_t	seq;
 
-	struct { /* extract of a clocksource struct */
-		int vclock_mode;
-		u64	cycle_last;
-		u64	mask;
-		u32	mult;
-		u32	shift;
-	} clock;
+	struct pvclock_clock clock; /* extract of a clocksource struct */
+	struct pvclock_clock raw_clock; /* extract of a clocksource struct */
 
+	u64		boot_ns_raw;
 	u64		boot_ns;
 	u64		nsec_base;
 	u64		wall_time_sec;
+	u64		monotonic_raw_nsec;
 };
 
 static struct pvclock_gtod_data pvclock_gtod_data;
@@ -1261,10 +1266,20 @@
 static void update_pvclock_gtod(struct timekeeper *tk)
 {
 	struct pvclock_gtod_data *vdata = &pvclock_gtod_data;
-	u64 boot_ns;
+	u64 boot_ns, boot_ns_raw;
 
 	boot_ns = ktime_to_ns(ktime_add(tk->tkr_mono.base, tk->offs_boot));
 
+	/*
+	 * FIXME: tk->offs_boot should be converted to CLOCK_MONOTONIC_RAW
+	 * interval (that is, without frequency adjustment for that interval).
+	 *
+	 * Lack of this fix can cause system_timestamp to not be equal to
+	 * CLOCK_MONOTONIC_RAW (which happen if the host uses
+	 * suspend/resume).
+	 */
+	boot_ns_raw = ktime_to_ns(ktime_add(tk->tkr_raw.base, tk->offs_boot));
+
 	write_seqcount_begin(&vdata->seq);
 
 	/* copy pvclock gtod data */
@@ -1274,11 +1289,20 @@
 	vdata->clock.mult		= tk->tkr_mono.mult;
 	vdata->clock.shift		= tk->tkr_mono.shift;
 
+	vdata->raw_clock.vclock_mode	= tk->tkr_raw.clock->archdata.vclock_mode;
+	vdata->raw_clock.cycle_last	= tk->tkr_raw.cycle_last;
+	vdata->raw_clock.mask		= tk->tkr_raw.mask;
+	vdata->raw_clock.mult		= tk->tkr_raw.mult;
+	vdata->raw_clock.shift		= tk->tkr_raw.shift;
+
 	vdata->boot_ns			= boot_ns;
 	vdata->nsec_base		= tk->tkr_mono.xtime_nsec;
 
 	vdata->wall_time_sec            = tk->xtime_sec;
 
+	vdata->boot_ns_raw		= boot_ns_raw;
+	vdata->monotonic_raw_nsec	= tk->tkr_raw.xtime_nsec;
+
 	write_seqcount_end(&vdata->seq);
 }
 #endif
@@ -1713,21 +1737,21 @@
 	return last;
 }
 
-static inline u64 vgettsc(u64 *tsc_timestamp, int *mode)
+static inline u64 vgettsc(struct pvclock_clock *clock, u64 *tsc_timestamp,
+			  int *mode)
 {
 	long v;
-	struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
 	u64 tsc_pg_val;
 
-	switch (gtod->clock.vclock_mode) {
+	switch (clock->vclock_mode) {
 	case VCLOCK_HVCLOCK:
 		tsc_pg_val = hv_read_tsc_page_tsc(hv_get_tsc_page(),
 						  tsc_timestamp);
 		if (tsc_pg_val != U64_MAX) {
 			/* TSC page valid */
 			*mode = VCLOCK_HVCLOCK;
-			v = (tsc_pg_val - gtod->clock.cycle_last) &
-				gtod->clock.mask;
+			v = (tsc_pg_val - clock->cycle_last) &
+				clock->mask;
 		} else {
 			/* TSC page invalid */
 			*mode = VCLOCK_NONE;
@@ -1736,8 +1760,8 @@
 	case VCLOCK_TSC:
 		*mode = VCLOCK_TSC;
 		*tsc_timestamp = read_tsc();
-		v = (*tsc_timestamp - gtod->clock.cycle_last) &
-			gtod->clock.mask;
+		v = (*tsc_timestamp - clock->cycle_last) &
+			clock->mask;
 		break;
 	default:
 		*mode = VCLOCK_NONE;
@@ -1746,10 +1770,10 @@
 	if (*mode == VCLOCK_NONE)
 		*tsc_timestamp = v = 0;
 
-	return v * gtod->clock.mult;
+	return v * clock->mult;
 }
 
-static int do_monotonic_boot(s64 *t, u64 *tsc_timestamp)
+static int do_monotonic_raw(s64 *t, u64 *tsc_timestamp)
 {
 	struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
 	unsigned long seq;
@@ -1758,10 +1782,10 @@
 
 	do {
 		seq = read_seqcount_begin(&gtod->seq);
-		ns = gtod->nsec_base;
-		ns += vgettsc(tsc_timestamp, &mode);
+		ns = gtod->monotonic_raw_nsec;
+		ns += vgettsc(&gtod->raw_clock, tsc_timestamp, &mode);
 		ns >>= gtod->clock.shift;
-		ns += gtod->boot_ns;
+		ns += gtod->boot_ns_raw;
 	} while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
 	*t = ns;
 
@@ -1779,7 +1803,7 @@
 		seq = read_seqcount_begin(&gtod->seq);
 		ts->tv_sec = gtod->wall_time_sec;
 		ns = gtod->nsec_base;
-		ns += vgettsc(tsc_timestamp, &mode);
+		ns += vgettsc(&gtod->clock, tsc_timestamp, &mode);
 		ns >>= gtod->clock.shift;
 	} while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
 
@@ -1796,7 +1820,7 @@
 	if (!gtod_is_based_on_tsc(pvclock_gtod_data.clock.vclock_mode))
 		return false;
 
-	return gtod_is_based_on_tsc(do_monotonic_boot(kernel_ns,
+	return gtod_is_based_on_tsc(do_monotonic_raw(kernel_ns,
 						      tsc_timestamp));
 }