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

Paolo Bonzini
2014-09-04T17:56:35Z

Il 04/09/2014 14:58, Paolo Bonzini ha scritto:
> Commit cbcf2dd3b3d4 (x86: kvm: Make kvm_get_time_and_clockread() nanoseconds
> based, 2014-07-16) forgot to add tk->xtime_sec, thus breaking kvmclock on
> hosts that have a reliable TSC. Add it back; and since the field boot_ns
> is not anymore related to the host boot-based clock, rename boot_ns->nsec_base
> and the existing nsec_base->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>
> ---
> arch/x86/kvm/x86.c | 11 ++++++-----
> 1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8f1e22d3b286..92493e10937c 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;
> @@ -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;

Hmm, I found this comment in kernel/time/timekeeping.c

/*
* The xtime based monotonic readout is:
* nsec = (xtime_sec + wtm_sec) * 1e9 + wtm_nsec + now();
* The ktime based monotonic readout is:
* nsec = base_mono + now();

so this patch makes no sense. The offs_boot part must be broken.

Paolo

>
> 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;
>
>