Re: [PATCH RFC 2/6] x86/time: implement tsc as clocksource

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 28/12/2015 16:59, 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 nothing that with clocksource=tsc there isn't no

I presume you mean "worth noting" and either "is no" or "isn't any"?

However, looking at the sentence below, do you mean "isn't any need to 
synchronise with another clocksource" ?

> synchronization 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 used to observe ~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@oracle.com>
> ---
>   xen/arch/x86/time.c | 63 +++++++++++++++++++++++++++++++++++++++++++++++++++--
>   1 file changed, 61 insertions(+), 2 deletions(-)
>
> diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
> index 30d52c4..c9e5c14 100644
> --- a/xen/arch/x86/time.c
> +++ b/xen/arch/x86/time.c
> @@ -433,6 +433,64 @@ uint64_t ns_to_acpi_pm_tick(uint64_t ns)
>   }
>   
>   /************************************************************
> + * PLATFORM TIMER 4: TSC
> + */
> +static bool_t clocksource_is_tsc = 0;

What does clocksource_is_tsc signify? It looks to be unused in this patch.

~Andrew