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

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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