Re: lock in vhpet

["Andres Lagar-Cavilla" <andres@lagarcavilla.org>]

>
>> -----Original Message-----
>> From: Andres Lagar-Cavilla [mailto:andres@lagarcavilla.org]
>> Sent: Wednesday, April 25, 2012 9:40 AM
>> To: Zhang, Yang Z
>> Cc: Tim Deegan; xen-devel@lists.xensource.com; Keir Fraser
>> Subject: RE: [Xen-devel] lock in vhpet
>>
>> >> -----Original Message-----
>> >> From: Tim Deegan [mailto:tim@xen.org]
>> >> Sent: Tuesday, April 24, 2012 5:17 PM
>> >> To: Zhang, Yang Z
>> >> Cc: andres@lagarcavilla.org; xen-devel@lists.xensource.com; Keir
>> >> Fraser
>> >> Subject: Re: [Xen-devel] lock in vhpet
>> >>
>> >> At 08:58 +0000 on 24 Apr (1335257909), Zhang, Yang Z wrote:
>> >> > > -----Original Message-----
>> >> > > From: Andres Lagar-Cavilla [mailto:andres@lagarcavilla.org]
>> >> > > Sent: Tuesday, April 24, 2012 1:19 AM
>> >> > >
>> >> > > Let me know if any of this helps
>> >> > No, it not works.
>> >>
>> >> Do you mean that it doesn't help with the CPU overhead, or that it's
>> >> broken in some other way?
>> >>
>> > It cannot help with the CPU overhead
>>
>> Yang, is there any further information you can provide? A rough idea of
>> where
>> vcpus are spending time spinning for the p2m lock would be tremendously
>> useful.
>>
> I am doing the further investigation. Hope can get more useful
> information.

Thanks, looking forward to that.

> But actually, the first cs introduced this issue is 24770. When win8
> booting and if hpet is enabled, it will use hpet as the time source and
> there have lots of hpet access and EPT violation. In EPT violation
> handler, it call get_gfn_type_access to get the mfn. The cs 24770
> introduces the gfn_lock for p2m lookups, and then the issue happens. After
> I removed the gfn_lock, the issue goes. But in latest xen, even I remove
> this lock, it still shows high cpu utilization.
>

It would seem then that even the briefest lock-protected critical section
would cause this? In the mmio case, the p2m lock taken in the hap fault
handler is held during the actual lookup, and for a couple of branch
instructions afterwards.

In latest Xen, with lock removed for get_gfn, on which lock is time spent?

Thanks,
Andres

> yang
>