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Re: [Xen-devel] Xen 4.3 development update

On Apr 3, 2013, at 6:53 AM, George Dunlap <george.dunlap@xxxxxxxxxxxxx> wrote:

> On 03/04/13 08:27, Jan Beulich wrote:
>>>>> On 02.04.13 at 18:34, Tim Deegan <tim@xxxxxxx> wrote:
>>> At 16:42 +0100 on 02 Apr (1364920927), Jan Beulich wrote:
>>>>>>> On 02.04.13 at 16:07, George Dunlap <George.Dunlap@xxxxxxxxxxxxx> wrote:
>>>>> * AMD NPT performance regression after c/s 24770:7f79475d3de7
>>>>>   owner: ?
>>>>>   Reference: http://marc.info/?l=xen-devel&m=135075376805215
>>>> This is supposedly fixed with the RTC changes Tim committed the
>>>> other day. Suravee, is that correct?
>>> This is a separate problem.  IIRC the AMD XP perf issue is caused by the
>>> emulation of LAPIC TPR accesses slowing down with Andres's p2m locking
>>> patches.  XP doesn't have 'lazy IRQL' or support for CR8, so it takes a
>>> _lot_ of vmexits for IRQL reads and writes.
>> Ah, okay, sorry for mixing this up. But how is this a regression
>> then?
> My sense, when I looked at this back whenever that there was much more to 
> this.  The XP IRQL updating is a problem, but it's made terribly worse by the 
> changset in question.  It seemed to me like the kind of thing that would be 
> caused by TLB or caches suddenly becoming much less effective.

The commit in question does not add p2m mutations, so it doesn't nuke the 
NPT/EPT TLBs. It introduces a spin lock in the hot path and that is the 
problem. Later in the 4.2 cycle we changed the common case to use an rwlock. 
Does the same perf degradation occur with tip of 4.2?

> -George

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