Re: [Xen-devel] [PATCH] x86/hvm/pmtimer: improving scalability of virtual time update on Xen 4.0.0

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

At 21:39 +0000 on 17 Nov (1290029945), Song Xiang wrote:
> diff --git a/xen/arch/x86/hvm/pmtimer.c b/xen/arch/x86/hvm/pmtimer.c
> index 48fe26a..587944f 100644
> --- a/xen/arch/x86/hvm/pmtimer.c
> +++ b/xen/arch/x86/hvm/pmtimer.c
> @@ -90,7 +90,7 @@ static void pmt_update_time(PMTState *s)
> 
>      /* Update the timer */
>      curr_gtime = hvm_get_guest_time(s->vcpu);
> -    s->pm.tmr_val += ((curr_gtime - s->last_gtime) * s->scale) >> 32;
> +    *(volatile uint32_t *)&s->pm.tmr_val = s->pm.tmr_val + (((curr_gtime - 
> s->last_gtime) * s->scale) >> 32);
>      s->pm.tmr_val &= TMR_VAL_MASK;
>      s->last_gtime = curr_gtime;

That doesn't make it an atomic update!  The variable is still written
to twice. :)  You need to calculate the new tmr_val in a scratch
variable, and then write it back once at the end of the function.
(And no 'volatile' wll be necessary).

Cheers,

Tim.

> @@ -206,10 +206,19 @@ static int handle_pmt_io(
> 
>      if ( dir == IOREQ_READ )
>      {
> -        spin_lock(&s->lock);
> -        pmt_update_time(s);
> -        *val = s->pm.tmr_val;
> -        spin_unlock(&s->lock);
> +        /*
> +         * if acquired the PMTState lock then update the time
> +         * else other vcpu is updating it ,it should be up to date.
> +         */
> +        if (spin_trylock(&s->lock)) {
> +            pmt_update_time(s);
> +            *val = s->pm.tmr_val;
> +            spin_unlock(&s->lock);
> +        }
> +        else {
> +            spin_barrier(&s->lock);
> +            *val = s->pm.tmr_val;
> +        }
>          return X86EMUL_OKAY;
>      }
> 

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-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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