RE: [Xen-devel] [PATCH]Fixes for overflowed calculation in vHPET

["Shan, Haitao" <haitao.shan@xxxxxxxxx>]

I think it is OK for normal usage and for 32bit timer operation.

But if a timer is programmed at 64bit mode, and the period programmed is sufficiently large, say 0xf000_0000_0000_0000, the code introduces trouble. Actually the timer should never be fired. However, (int64_s)0xf000_0000_0000_0000 < 0, then the period is forced to 0 and the timer is fired immediately.

Best Regards
Haitao Shan

 

---

From: Keir Fraser [mailto:Keir.Fraser@xxxxxxxxxxxx]
Sent: 2008年1月8日 22:15
To: Shan, Haitao; xen-devel@xxxxxxxxxxxxxxxxxxx
Cc: Mark McLoughlin; Cui, Dexuan
Subject: Re: [Xen-devel] [PATCH]Fixes for overflowed calculation in vHPET

On 4/1/08 03:21, "Shan, Haitao" 
<haitao.shan@xxxxxxxxx> wrote:

> This patch will 
>   fix the bugs in hpet_set_timer. Currently in hpet_tick_to_ns, the approach is 
>   multiplying first, which easily causes overflow when tick is quite large. The 
>   patch cannot handle arbitrate large ticks duo to the precision requirement and 
>   64bit's value range. But by optimize the equation, a larger ticks than current 
>   code can be supported. Also an overflow check is added before the calculation. 
>   
> This patch will also fix the wrong handling of wrap around case when timer 
>   is in 64bit mode. 
>   

What’s wrong with the handling of the 
wrap-around case? It looks okay to me.

 -- 
Keir
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