From: "Jan Beulich" <JBeulich@suse.com>
To: Joao Martins <joao.m.martins@oracle.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
Keir Fraser <keir@xen.org>,
xen-devel@lists.xen.org
Subject: Re: [PATCH v2 3/6] x86/time: implement tsc as clocksource
Date: Tue, 05 Apr 2016 04:43:44 -0600 [thread overview]
Message-ID: <5703B30002000078000E320D@prv-mh.provo.novell.com> (raw)
In-Reply-To: <1459259051-4943-4-git-send-email-joao.m.martins@oracle.com>
>>> On 29.03.16 at 15:44, <joao.m.martins@oracle.com> wrote:
> Introduce support for using TSC as platform time which is the highest
> resolution time and most performant to get (~20 nsecs). Though there
> are also several problems associated with its usage, and there isn't a
> complete (and architecturally defined) guarantee that all machines
> will provide reliable and monotonic TSC across all CPUs, on different
> sockets and different P/C states. I believe Intel to be the only that
> can guarantee that. For this reason it's only set when adminstrator
> changes "clocksource" boot option to "tsc". Initializing TSC
> clocksource requires all CPUs to have the tsc reliability checks
> performed. init_xen_time is called before all CPUs are up, and so we
> start with HPET at boot time, and switch later to TSC.
Please don't make possibly wrong statements: There's no guarantee
we'd start with HPET - might as well be PMTMR or PIT.
> The switch then
> happens on verify_tsc_reliability initcall that is invoked when all
> CPUs are up. When attempting to initializing TSC we also check for
> time warps and appropriate CPU features i.e. TSC_RELIABLE,
> CONSTANT_TSC and NONSTOP_TSC. And in case none of these conditions are
> met, we keep the clocksource that was previously initialized on
> init_xen_time.
DYM "And in case any of these conditions is not met, ..."? Or,
looking at the code, something more complex?
> --- a/xen/arch/x86/time.c
> +++ b/xen/arch/x86/time.c
> @@ -432,6 +432,63 @@ uint64_t ns_to_acpi_pm_tick(uint64_t ns)
> }
>
> /************************************************************
> + * PLATFORM TIMER 4: TSC
> + */
> +static u64 tsc_freq;
> +static unsigned long tsc_max_warp;
> +static void tsc_check_reliability(void);
> +
> +static int __init init_tsctimer(struct platform_timesource *pts)
> +{
> + bool_t tsc_reliable = 0;
> +
> + tsc_check_reliability();
> +
> + if ( tsc_max_warp > 0 )
> + {
> + tsc_reliable = 0;
Redundant assignment.
> +static void resume_tsctimer(struct platform_timesource *pts)
> +{
> +}
No need for an empty function - other clock sources don't have
such empty stubs either (i.e. the caller checks for NULL).
> @@ -541,6 +613,10 @@ static int __init try_platform_timer(struct platform_timesource *pts)
> if ( rc <= 0 )
> return rc;
>
> + /* We have a platform timesource already so reset it */
> + if ( plt_src.counter_bits != 0 )
> + reset_platform_timer();
What if any of the time functions get used between here and the
setting of the new clock source? For example, what will happen to
log messages when "console_timestamps=..." is in effect?
> @@ -566,7 +642,9 @@ static void __init init_platform_timer(void)
> struct platform_timesource *pts = NULL;
> int i, rc = -1;
>
> - if ( opt_clocksource[0] != '\0' )
> + /* clocksource=tsc is initialized later when all CPUS are up */
> + if ( (opt_clocksource[0] != '\0') &&
> + (strcmp(opt_clocksource, "tsc") != 0) )
In line with the inverse check done below (using ! instead of == 0)
please omit the != 0 here.
> @@ -1437,6 +1515,20 @@ static int __init verify_tsc_reliability(void)
> }
> }
>
> + if ( !strcmp(opt_clocksource, "tsc") )
> + {
> + if ( try_platform_timer(&plt_tsc) > 0 )
> + {
> + printk(XENLOG_INFO "Switched to Platform timer %s TSC\n",
> + freq_string(plt_src.frequency));
> +
> + init_percpu_time();
So you re-do this for the BP, but not for any of the APs?
Jan
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next prev parent reply other threads:[~2016-04-05 10:43 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-29 13:44 [PATCH v2 0/6] x86/time: PVCLOCK_TSC_STABLE_BIT support Joao Martins
2016-03-29 13:44 ` [PATCH v2 1/6] public/xen.h: add flags field to vcpu_time_info Joao Martins
2016-03-30 15:49 ` Ian Jackson
2016-03-30 16:33 ` Joao Martins
2016-03-31 7:09 ` Jan Beulich
2016-03-31 7:13 ` Jan Beulich
2016-03-31 11:04 ` Joao Martins
2016-04-05 10:16 ` Jan Beulich
2016-04-05 10:59 ` Joao Martins
2016-03-29 13:44 ` [PATCH v2 2/6] x86/time: refactor init_platform_time() Joao Martins
2016-04-01 16:10 ` Konrad Rzeszutek Wilk
2016-04-01 18:26 ` Joao Martins
2016-04-05 10:09 ` Jan Beulich
2016-04-05 10:55 ` Joao Martins
2016-04-05 11:16 ` Jan Beulich
2016-03-29 13:44 ` [PATCH v2 3/6] x86/time: implement tsc as clocksource Joao Martins
2016-03-29 17:39 ` Konrad Rzeszutek Wilk
2016-03-29 17:52 ` Joao Martins
2016-04-01 16:43 ` Konrad Rzeszutek Wilk
2016-04-01 18:38 ` Joao Martins
2016-04-01 18:45 ` Konrad Rzeszutek Wilk
2016-04-03 18:47 ` Joao Martins
2016-04-05 10:43 ` Jan Beulich [this message]
2016-04-05 14:56 ` Joao Martins
2016-04-05 15:12 ` Jan Beulich
2016-04-05 17:07 ` Joao Martins
2016-03-29 13:44 ` [PATCH v2 4/6] x86/time: streamline platform time init on plt_init() Joao Martins
2016-04-05 11:46 ` Jan Beulich
2016-04-05 15:12 ` Joao Martins
2016-04-05 15:22 ` Jan Beulich
2016-04-05 17:17 ` Joao Martins
2016-03-29 13:44 ` [PATCH v2 5/6] x86/time: refactor read_platform_stime() Joao Martins
2016-04-01 18:32 ` Konrad Rzeszutek Wilk
2016-04-05 11:52 ` Jan Beulich
2016-04-05 15:22 ` Joao Martins
2016-04-05 15:26 ` Jan Beulich
2016-04-05 17:08 ` Joao Martins
2016-03-29 13:44 ` [PATCH v2 6/6] x86/time: implement PVCLOCK_TSC_STABLE_BIT Joao Martins
2016-04-05 12:22 ` Jan Beulich
2016-04-05 21:34 ` Joao Martins
2016-04-07 15:58 ` Jan Beulich
2016-04-07 21:17 ` Joao Martins
2016-04-07 21:32 ` Jan Beulich
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