Re: [PATCH] Always save/restore performance counters when HVM guest switching VCPU

[George Dunlap <george.dunlap@eu.citrix.com>]

On 08/03/13 15:11, Boris Ostrovsky wrote:
> ----- george.dunlap@eu.citrix.com wrote:
>
>> On 08/03/13 14:50, Boris Ostrovsky wrote:
>>> ----- JBeulich@suse.com wrote:
>>>
>>>>>>> On 04.03.13 at 13:42, George Dunlap
>> <George.Dunlap@eu.citrix.com>
>>>> wrote:
>>>>> On Fri, Mar 1, 2013 at 8:49 PM,  <suravee.suthikulpanit@amd.com>
>>>> wrote:
>>>>>> From: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
>>>>>>
>>>>>> Currently, the performance counter registers are saved/restores
>>>>>> when the HVM guest switchs VCPUs only if they are running.
>>>>>> However, PERF has one check where it writes the MSR and read
>> back
>>>>>> the value to check if the MSR is working.  This has shown to
>> fails
>>>>>> the check if the VCPU is moved in between rdmsr and wrmsr and
>>>>>> resulting in the values are different.
>>>>> Many moons ago (circa 2005) when I used performance counters, I
>>>> found
>>>>> that adding them to the save/restore path added a non-neligible
>>>>> overhead -- something like 5% slow-down.  Do you have any reason
>> to
>>>>> believe this is no longer the case?  Have you done any benchmarks
>>>>> before and after?
>>> I was doing some VPMU tracing a couple of weeks ago and by looking
>> at
>>> trace timestamps I think I saw about 4000 cycles on VPMU save and
>>> ~9000 cycles on restore. Don't remember what it was percentage-wise
>> of
>>> a whole context switch.
>>>
>>> This was on Intel.
>> That's a really hefty expense to make all users pay on every context
>> switch, on behalf of a random check in a piece of software that only a
>> handful of people are going to be actually using.
> I believe Linux uses perf infrastructure to implement the watchdog.

Hmm -- well if it is the case that adding performance counters to the 
vcpu context switch path will add a measurable overhead, then we 
probably don't want them enabled for typical guests anyway.  If people 
are actually using the performance counters to measure performance, that 
makes sense; but for watchdogs it seems like Xen should be able to 
provide something that is useful for a watchdog without the extra 
overhead of saving and restoring performance counters.

Konrad, any thoughts?

  -George