Re: [Xen-devel] [PATCH 2/5] x86/time: implement tsc as clocksource

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 17/03/16 16:12, Joao Martins 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 set with less priority when
> compared to HPET unless adminstrator changes "clocksource" boot option
> to "tsc". Upon initializing it, 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 fail in
> order to fallback to the next available clocksource.
>
> It is also worth noting that with clocksource=tsc there isn't any
> need to synchronise with another clocksource, and I could verify that
> great portion the time skew was eliminated and seeing much less time
> warps happening. With HPET I observed ~500 warps in the period
> of 1h of around 27 us, and with TSC down to 50 warps in the same
> period having each warp < 100 ns. The warps still exist though but
> are only related to cross CPU calibration (being how much it takes to
> rendezvous with master), in which a later patch in this series aims to
> solve.
>
> Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
> ---
> Cc: Keir Fraser <keir@xxxxxxx>
> Cc: Jan Beulich <jbeulich@xxxxxxxx>
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Some style corrections, but no functional problems.

Reviewed-by Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

>
> Changes since RFC:
>  - Spelling fixes in the commit message.
>  - Remove unused clocksource_is_tsc variable and introduce it instead
>  on the patch that uses it.
>  - Move plt_tsc from second to last in the available clocksources.
> ---
>  xen/arch/x86/time.c | 62 
> +++++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 60 insertions(+), 2 deletions(-)
>
> diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
> index 687e39b..1311c58 100644
> --- 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;
> +        printk("TSC: didn't passed warp test\n");

printk(XENLOG_INFO "TSC ...

> +    }
> +    else if ( boot_cpu_has(X86_FEATURE_TSC_RELIABLE) ||
> +              ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
> +                boot_cpu_has(X86_FEATURE_NONSTOP_TSC) ) )

You don't need extra spaces on inner brackets, so

boot_cpu_has(X86_FEATURE_TSC_RELIABLE) ||
(boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
 boot_cpu_has(X86_FEATURE_NONSTOP_TSC)) )

is fine.

> +    {
> +        tsc_reliable = 1;
> +    }
> +    else if ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) )
> +    {
> +        tsc_reliable = (max_cstate <= 2);
> +
> +        if (tsc_reliable)

Please add some extra ones here though.

> +            printk("TSC: no deep Cstates, deemed reliable\n");
> +        else
> +            printk("TSC: deep Cstates possible, so not reliable\n");

XENLOG_INFO as well please.

~Andrew

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