Re: The HPET issue on Linux

[Anthony Liguori <anthony@codemonkey.ws>]

On 01/06/2010 01:44 PM, Gleb Natapov wrote:
> On Wed, Jan 06, 2010 at 01:23:28PM -0600, Anthony Liguori wrote:
>    
>> On 01/06/2010 01:20 PM, Beth Kon wrote:
>>      
>>> Beth Kon wrote:
>>>        
>>>> I will try to look into this. Since HPET is edge-triggered,
>>>> looks like this problem is of a different nature than PIT.  Is
>>>> this a solid failure or intermittent?
>>>>          
>>> Anthony just explained that on x86, even edge-triggered interrupts
>>> are queued in the apic and an eoi will occur, so this is not
>>> different than the PIT.
>>>        
>> Not quite queued in the sense that multiple events will be delivered
>> in order, but I think the point is that you can still detect whether
>> delivery succeeded by counting APIC EOIs.
>>
>> The trouble is that historically we've struggled with doing this in
>> userspace.  Maybe it's time to revisit.
>>
>>      
> We reinject PIT interrupts from kernel and RTC interrupts from
> userspace.
>    

Because we can determine that we've missed an RTC interrupt in 
userspace.  We cannot determine this with the PIT without adding a hook 
into the userspace apic that lets us know whether an injection failed or 
not.

Regards,

Anthony Liguori