[PATCH] KVM: x86: fix kvmclock breakage from timers branch merge

Paolo Bonzini
2014-09-04T21:05:44Z

Commit cbcf2dd3b3d4 (x86: kvm: Make kvm_get_time_and_clockread() nanoseconds
based, 2014-07-16) used the wrong formula for boot_ns, thus breaking kvmclock on
hosts that have a reliable TSC.

To find the right formula, let's first backport the switch to nanoseconds
to 3.16-era timekeeping logic. The full patch (which works) is at
https://lkml.org/lkml/2014/9/4/462. The key line here is

boot_ns = timespec_to_ns(&tk->total_sleep_time)
+ timespec_to_ns(&tk->wall_to_monotonic)
+ tk->xtime_sec * (u64)NSEC_PER_SEC;

Because the above patch works, the conclusion is that the above formula
is not the same as commit cbcf2dd3b3d4's

boot_ns = ktime_to_ns(ktime_add(tk->tkr.base_mono, tk->offs_boot));

As to what is the right one, commit 02cba1598a2a (timekeeping: Simplify getboottime(),
2014-07-16) provides a hint:

offs_real = -wall-to_monotonic
offs_boot = total_sleep_time

offs_real - offs_boot = -wall_to_monotonic - total_sleep_time

that is

offs_boot - offs_real = wall_to_monotonic + total_sleep_time

which is what this patch uses, adding xtime_sec separately. The "boot_ns"
moniker is not too clear, so rename boot_ns to nsec_base and the existing
nsec_base to snsec_base.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Reported-by: Chris J Arges <chris.j.arges@canonical.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
Thomas/John, the problem with the above explanation is that
tk_update_ktime_data has "base_mono = xtime_sec + wtm", and from
there "base_mono + offs_boot = xtime_sec + wtm + total_sleep_time".
Except that doesn't work, so something must be wrong in
tk_update_ktime_data's comment.

arch/x86/kvm/x86.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 8f1e22d3b286..c55203bea337 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1020,8 +1020,8 @@ struct pvclock_gtod_data {
u32 shift;
} clock;

- u64 boot_ns;
u64 nsec_base;
+ u64 snsec_base;
};

static struct pvclock_gtod_data pvclock_gtod_data;
@@ -1031,7 +1031,7 @@ static void update_pvclock_gtod(struct timekeeper *tk)
struct pvclock_gtod_data *vdata = &pvclock_gtod_data;
u64 boot_ns;

- boot_ns = ktime_to_ns(ktime_add(tk->tkr.base_mono, tk->offs_boot));
+ boot_ns = ktime_to_ns(ktime_sub(tk->tkr.offs_boot, tk->offs_real));

write_seqcount_begin(&vdata->seq);

@@ -1042,8 +1042,9 @@ static void update_pvclock_gtod(struct timekeeper *tk)
vdata->clock.mult = tk->tkr.mult;
vdata->clock.shift = tk->tkr.shift;

- vdata->boot_ns = boot_ns;
- vdata->nsec_base = tk->tkr.xtime_nsec;
+ vdata->nsec_base = tk->xtime_sec * (u64)NSEC_PER_SEC
+ + boot_ns;
+ vdata->snsec_base = tk->tkr.xtime_nsec;

write_seqcount_end(&vdata->seq);
}
@@ -1413,10 +1414,10 @@ static int do_monotonic_boot(s64 *t, cycle_t *cycle_now)
do {
seq = read_seqcount_begin(&gtod->seq);
mode = gtod->clock.vclock_mode;
- ns = gtod->nsec_base;
+ ns = gtod->snsec_base;
ns += vgettsc(cycle_now);
ns >>= gtod->clock.shift;
- ns += gtod->boot_ns;
+ ns += gtod->nsec_base;
} while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
*t = ns;

--
1.8.3.1