[SPAM] Re: [Xen-devel] A patch to xen/arch/x86/hvm/pmtimer.c for both Xen 4.0.0 and Xen 4.0.1 to improve HVM scalability

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> On 16.11.10 at 15:51, Song Xiang <classicxsong@xxxxxxxxx> wrote:
> We updated our patch as follows:
> 
> Index: arch/x86/hvm/pmtimer.c
> ===================================================================
> --- arch/x86/hvm/pmtimer.c
> +++ arch/x86/hvm/pmtimer.c
> @@ -206,13 +206,23 @@
> 
>       if ( dir == IOREQ_READ )
>       {
> -        spin_lock(&s->lock);
> -        pmt_update_time(s);
> -        *val = s->pm.tmr_val;
> -        spin_unlock(&s->lock);
> +        uint32_t tmp;
> +        /*
> +         * if acquired the PMTState lock then update the time
> +         * else other vcpu is updating it ,it should be up to date.
> +         */
> +        tmp = atomic_read(&s-> ownership);
> +        if (spin_trylock(&s->lock)) {
> +            pmt_update_time(s);
> +            *val = s->pm.tmr_val;
> +            spin_unlock(&s->lock);
> +            atomic_inc(&s-> ownership);
> +        }
> +        else {
> +            while (tmp == atomic_read(&s-> ownership));
> +
> +           *val = s->pm.tmr_val;
> +        }
>           return X86EMUL_OKAY;
>       }
> 
> Index: include/asm-x86/hvm/vpt.h
> ===================================================================
> --- include/asm-x86/hvm/vpt.h
> +++ include/asm-x86/hvm/vpt.h
> @@ -120,6 +120,7 @@
>       uint64_t scale;             /* Multiplier to get from tsc to  
> timer ticks */
>       struct timer timer;         /* To make sure we send SCIs */
>       spinlock_t lock;
> +   atomic_t ownership;
>   } PMTState;
> 
>   struct pl_time {    /* platform time */
> 
> 
> The key idea is, to keep the returned time fresh, any VCPU that fails  
> to acquire the PMTState lock will wait until the PMTState lock holder  
> updates the global virtual time, and then returns the freshest time.
>
> We add a field in PMTState, named ownership. The PMTState->ownership  
> can only be updated by the PMTState lock holder.It is updated after  
> the PMTState lock holder has updated the global virtual time and  
> released the MTState lock.  Other VCPUs that fail to acquire the  
> PMTState lock will check whether the MTState->ownership is updated in  
> a while loop.  When the PMTState->ownership is changed, the global  
> virtual time must be the freshest, and can be returned to the guest OS.
> 
> The time returned to the guest in this patch is fresher than the  
> previous one we have proposed.

That's not better: Depending on timing, the reader may well loop
until the *next* acquirer releases the lock (and increments
"ownership").

Jan


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