Re: [Xen-devel] [PATCH RFC v1 30/74] x86: APIC timer calibration when running as a guest

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.01.18 at 14:05, <wei.liu2@xxxxxxxxxx> wrote:
> The timer calibration depends on the number of ticks. Introduce a
> variant to wait for a tick when running as a guest.

The change itself is fine, i.e.
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
but the description (to me, but it may be just me) doesn't really
match it. How about

The timer calibration currently depends on PIT. Introduce a variant
to wait for a tick's worth of time to elapse when running as a PVH
guest.

Jan

> Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> ---
>  xen/arch/x86/apic.c | 38 ++++++++++++++++++++++++++++++--------
>  1 file changed, 30 insertions(+), 8 deletions(-)
> 
> diff --git a/xen/arch/x86/apic.c b/xen/arch/x86/apic.c
> index ed59440c45..5039173827 100644
> --- a/xen/arch/x86/apic.c
> +++ b/xen/arch/x86/apic.c
> @@ -36,6 +36,8 @@
>  #include <mach_apic.h>
>  #include <io_ports.h>
>  #include <xen/kexec.h>
> +#include <asm/guest.h>
> +#include <asm/time.h>
>  
>  static bool __read_mostly tdt_enabled;
>  static bool __initdata tdt_enable = true;
> @@ -1091,6 +1093,20 @@ static void setup_APIC_timer(void)
>      local_irq_restore(flags);
>  }
>  
> +static void wait_tick_pvh(void)
> +{
> +    u64 lapse_ns = 1000000000ULL / HZ;
> +    s_time_t start, curr_time;
> +
> +    start = NOW();
> +
> +    /* Won't wrap around */
> +    do {
> +        cpu_relax();
> +        curr_time = NOW();
> +    } while ( curr_time - start < lapse_ns );
> +}
> +
>  /*
>   * In this function we calibrate APIC bus clocks to the external
>   * timer. Unfortunately we cannot use jiffies and the timer irq
> @@ -1123,12 +1139,15 @@ static int __init calibrate_APIC_clock(void)
>       */
>      __setup_APIC_LVTT(1000000000);
>  
> -    /*
> -     * The timer chip counts down to zero. Let's wait
> -     * for a wraparound to start exact measurement:
> -     * (the current tick might have been already half done)
> -     */
> -    wait_8254_wraparound();
> +    if ( !xen_guest )
> +        /*
> +         * The timer chip counts down to zero. Let's wait
> +         * for a wraparound to start exact measurement:
> +         * (the current tick might have been already half done)
> +         */
> +        wait_8254_wraparound();
> +    else
> +        wait_tick_pvh();
>  
>      /*
>       * We wrapped around just now. Let's start:
> @@ -1137,10 +1156,13 @@ static int __init calibrate_APIC_clock(void)
>      tt1 = apic_read(APIC_TMCCT);
>  
>      /*
> -     * Let's wait LOOPS wraprounds:
> +     * Let's wait LOOPS ticks:
>       */
>      for (i = 0; i < LOOPS; i++)
> -        wait_8254_wraparound();
> +        if ( !xen_guest )
> +            wait_8254_wraparound();
> +        else
> +            wait_tick_pvh();
>  
>      tt2 = apic_read(APIC_TMCCT);
>      t2 = rdtsc_ordered();
> -- 
> 2.11.0
> 
> 
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