Re: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

On 07/11/2013 04:26 PM, Gleb Natapov wrote:
> On Thu, Jul 11, 2013 at 04:23:58PM +0530, Raghavendra K T wrote:
>> On 07/11/2013 03:41 PM, Gleb Natapov wrote:
>>> On Thu, Jul 11, 2013 at 03:40:38PM +0530, Raghavendra K T wrote:
>>>>>>>> Gleb,
>>>>>>>> Can you elaborate little more on what you have in mind regarding per
>>>>>>>> VM ple_window. (maintaining part of it as a per vm variable is clear
>>>>>>>> to
>>>>>>>>   me), but is it that we have to load that every time of guest entry?
>>>>>>>>
>>>>>>> Only when it changes, shouldn't be to often no?
>>>>>>
>>>>>> Ok. Thinking how to do. read the register and writeback if there need
>>>>>> to be a change during guest entry?
>>>>>>
>>>>> Why not do it like in the patch you've linked? When value changes write it
>>>>> to VMCS of the current vcpu.
>>>>>
>>>>
>>>> Yes. can be done. So the running vcpu's ple_window gets updated only
>>>> after next pl-exit. right?
>>> I am not sure what you mean. You cannot change vcpu's ple_window while
>>> vcpu is in a guest mode.
>>>
>>
>> I agree with that. Both of us are on the same page.
>>   What I meant is,
>> suppose the per VM ple_window changes when a vcpu x of that VM  was running,
>> it will get its ple_window updated during next run.
> Ah, I think "per VM" is what confuses me. Why do you want to have "per
> VM" ple_windows and not "per vcpu" one? With per vcpu one ple_windows
> cannot change while vcpu is running.
>

Okay. Got that. My initial feeling was vcpu does not "feel" the global
load. But I think that should be of no problem. instead we will not need
atomic operations to update ple_window, which is better.