Re: The HPET issue on Linux

[Dor Laor <dlaor@redhat.com>]

On 01/06/2010 12:09 PM, Gleb Natapov wrote:
> On Wed, Jan 06, 2010 at 05:48:52PM +0800, Sheng Yang wrote:
>> Hi Beth
>>
>> I still found the emulated HPET would result in some boot failure. For
>> example, on my 2.6.30, with HPET enabled, the kernel would fail check_timer(),
>> especially in timer_irq_works().
>>
>> The testing of timer_irq_works() is let 10 ticks pass(using mdelay()), and
>> want to confirm the clock source with at least 5 ticks advanced in jiffies.
>> I've checked that, on my machine, it would mostly get only 4 ticks when HPET
>> enabled, then fail the test. On the other hand, if I using PIT, it would get
>> more than 10 ticks(maybe understandable if some complementary ticks there). Of
>> course, extend the ticks count/mdelay() time can work.
>>
>> I think it's a major issue of HPET. And it maybe just due to a too long
>> userspace path for interrupt injection... If it's true, I think it's not easy
>> to deal with it.
>>
> PIT tick are reinjected automatically, HPET should probably do the same
> although it may just create another set of problems.

Older Linux do automatic adjustment for lost ticks so automatic 
reinjection causes time to run too fast. This is why we added the 
-no-kvm-pit-reinject flag...

It took lots of time to pit/rtc to stabilize, in order of seriously 
consider the hpet emulation, lots of testing should be done.

>
> --
> 			Gleb.
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