[Xen-devel] RE: Changeset 15943 broke Linux 2.6.23-rc4?

["Cui, Dexuan" <dexuan.cui@xxxxxxxxx>]

Yes, the newly-checked-in patch fixes the 2.6.23 issue.

Thanks!
-- Dexuan

Tim Deegan wrote:
> Hi,
> 
> At 18:53 +0800 on 17 Oct (1192647209), Cui, Dexuan wrote:
>> What's the meaning of "division by zero" in the change log of
>> changeset 15943 : c0d1825f5189 (Don't count "missed ticks" on
>> one-shot timers.)? 
> 
> An OS that set up a one-shot ACPI timer could cause the timer to fire
> with pt->period set to zero, which crashes Xen in the missed_ticks
> calculation. (vpt.c:56  missed_ticks = missed_ticks / (s_time_t)
> pt->period + 1;) Also, it's surely wrong to calculate "missed" ticks
> on a non-repeating timer.
> 
>> I found the c/s breaks Linux 2.6.23-rc4 when ACPI=1 in HVM config
>> file. 
>> 
>> I don't think the 2 lines below are correct for one_shot vpt:       
>>          pt->enabled = 0; list_del(&pt->list);
>> because i.e., it may drop one-shot local timer interrupt wrongly
>> (this breaks ACPI Linux 2.6.23-rc4...):
>> 1)   an one-shot timer interrupt is triggered in pt_timer_fn(), then
>> c/s 15943
>> sets pt->enabled to 0, and removes the vpt from the tm_list;
>> 2)   in vmx_intr_assit() - > pt_update_irq(), we can't find the pt in
>> the tm_list, so the timer interrupt is dropped...
> 
> Ah, I see.  Yes, those lines need to move to after the interrupt is
> delivered. :)
> 
>> Actually we don't need to remove an one_shot vpt from tm_list, since
>> pt_update_irq() ignores a vpt if pt->pending_intr_nr == 0.
> 
> We should do it, though, because otherwise we're just making
> pt_update_irq's list walk more expensive for no benefit.
> 
> Cheers,
> 
> Tim.

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