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@xxxxxxxxxxxxx> Cc: John Stultz <john.stultz@xxxxxxxxxx> Reported-by: Chris J Arges <chris.j.arges@xxxxxxxxxxxxx> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> --- 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(>od->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(>od->seq, seq))); *t = ns; -- 1.8.3.1 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
