Re: [patch 1/3] KVM: x86: provide realtime host clock via vsyscall notifiers

Radim Krcmar
2017-01-13T15:18:04Z

2017-01-13 10:01-0200, Marcelo Tosatti:
> Expose the realtime host clock and save the TSC value
> used for the clock calculation.
>
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
>
> ---
> arch/x86/kvm/x86.c | 38 ++++++++++++++++++++++++++++++++++++++
> 1 file changed, 38 insertions(+)
>
> Index: kvm-ptpdriver/arch/x86/kvm/x86.c
> ===================================================================
> --- kvm-ptpdriver.orig/arch/x86/kvm/x86.c 2017-01-13 08:59:03.015895353 -0200
> +++ kvm-ptpdriver/arch/x86/kvm/x86.c 2017-01-13 09:04:46.581415259 -0200
> @@ -1139,6 +1139,8 @@
>
> u64 boot_ns;
> u64 nsec_base;
> + u64 wall_time_sec;
> + u64 wall_time_snsec;

The leading "s" in "snsec" looks like a copy-paste residue.

> };
>
> static struct pvclock_gtod_data pvclock_gtod_data;
> @@ -1162,6 +1164,9 @@
> vdata->boot_ns = boot_ns;
> vdata->nsec_base = tk->tkr_mono.xtime_nsec;
>
> + vdata->wall_time_sec = tk->xtime_sec;
> + vdata->wall_time_snsec = tk->tkr_mono.xtime_nsec;

Using tk->tkr_mono offsets for real time seems wrong -- what happens if
the real time is half a second shifted from monotonic time?

If it's ok, then vdata->nsec_base == vdata->wall_time_snsec, so we don't
need it.

> +
> write_seqcount_end(&vdata->seq);
> }
> #endif
> @@ -1623,6 +1628,28 @@
> return mode;
> }
>
> +static int do_realtime(struct timespec *ts, cycle_t *cycle_now)

This is too similar to do_monotonic_boot(), but I don't see a solution
that is both nice and efficient. :(

(It usually means macros or copying pvclock_gtod_data.)

> +{
> + struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
> + unsigned long seq;
> + int mode;
> + u64 ns;
> +
> + do {
> + seq = read_seqcount_begin(&gtod->seq);
> + mode = gtod->clock.vclock_mode;
> + ts->tv_sec = gtod->wall_time_sec;
> + ns = gtod->wall_time_snsec;
> + ns += vgettsc(cycle_now);
> + ns >>= gtod->clock.shift;
> + } while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
> +
> + ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
> + ts->tv_nsec = ns;
> +
> + return mode;
> +}
> +
> /* returns true if host is using tsc clocksource */
> static bool kvm_get_time_and_clockread(s64 *kernel_ns, cycle_t *cycle_now)
> {
> @@ -1632,6 +1659,17 @@
>
> return do_monotonic_boot(kernel_ns, cycle_now) == VCLOCK_TSC;
> }
> +
> +/* returns true if host is using tsc clocksource */
> +static bool kvm_get_walltime_and_clockread(struct timespec *ts,
> + cycle_t *cycle_now)
> +{
> + /* checked again under seqlock below */
> + if (pvclock_gtod_data.clock.vclock_mode != VCLOCK_TSC)
> + return false;
> +
> + return do_realtime(ts, cycle_now) == VCLOCK_TSC;
> +}
> #endif
>
> /*
>
>