Re: [PATCH 0/8] use jump labels to streamline common APIC configuration

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-08-14 16:03, Gleb Natapov wrote:
> On Tue, Aug 14, 2012 at 04:00:54PM +0200, Jan Kiszka wrote:
>> On 2012-08-05 16:03, Gleb Natapov wrote:
>>> On Sun, Aug 05, 2012 at 05:00:37PM +0300, Avi Kivity wrote:
>>>> On 08/05/2012 04:48 PM, Gleb Natapov wrote:
>>>>  >>
>>>>>>>> During guest boot up, some of these jump keys will change, no?  Does
>>>>>>>> this mean a stop_machine() or equivalent?  I'm worried about real-time
>>>>>>>> response or one guest being affected by another.
>>>>>>>>
>>>>>>> Yes, SW enable bit changes during boot. The jump label triggerable by a
>>>>>>> guest are rate limited though. So stop machine will not happen more then
>>>>>>> once per second even with malicious guests.
>>>>>>
>>>>>> I'm not talking about a malicious guest, just a guest that is booting up
>>>>>> normally but kills real-time response for another guest (or just induces
>>>>>> a large hiccup in a non-real-time guest, but we don't guarantee anything
>>>>>> for those).
>>>>>>
>>>>>> We don't support real-time guests now, but Jan has plans.
>>>>>>
>>>>> For such setup jump labels have to be compiled out from the kernel
>>>>> completely. Anything that calls stop_machine does not play well with
>>>>> real time.
>>>>>
>>>>> Guest can cause stop machine on boot today already by detecting PMU and
>>>>> configuring NMI watchdog.
>>>>
>>>> The host can prevent this by leaving disabling the guest pmu.  But
>>>> disabling jump labels for real-time kernels may be acceptable too.  We
>>>> can probably to it at run time by forcing the slow path at all times.
>>> Yes, it is possible to add module option that will force slow path if
>>> needed.
>>
>> Should I write a patch or will you? Having host-side stop_machine due to
>> such common guest operations is indeed a no-go for RT.
>>
> Do we support RT now? The operation are definitely not common.

We also have min_timer_period_us to control the APIC timer - for the
same use case.

And regarding how common they are: Do standard OSes trigger any
jump-label optimized switch during at least their boot-up? I thought so.
In that case, if you co-locate RT and standard OSes on a shared host,
you would have a conflict.

Jan

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