Re: [Xen-devel] Ping: [PATCH] VMX: sync CPU state upon vCPU destruction

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

On 21/11/17 15:29, Jan Beulich wrote:
>>>> On 21.11.17 at 15:07, <igor.druzhinin@xxxxxxxxxx> wrote:
>> On 21/11/17 13:22, Jan Beulich wrote:
>>>>>> On 09.11.17 at 15:49, <JBeulich@xxxxxxxx> wrote:
>>>> See the code comment being added for why we need this.
>>>>
>>>> Reported-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>
>>> I realize we aren't settled yet on where to put the sync call. The
>>> discussion appears to have stalled, though. Just to recap,
>>> alternatives to the placement below are
>>> - at the top of complete_domain_destroy(), being the specific
>>>   RCU callback exhibiting the problem (others are unlikely to
>>>   touch guest state)
>>> - in rcu_do_batch(), paralleling the similar call from
>>>   do_tasklet_work()
>>
>> rcu_do_batch() sounds better to me. As I said before I think that the
>> problem is general for the hypervisor (not for VMX only) and might
>> appear in other places as well.
> 
> The question here is: In what other cases do we expect an RCU
> callback to possibly touch guest state? I think the common use is
> to merely free some memory in a delayed fashion.
> 

I don't know for sure what the common scenario is for Xen but drawing
parallels between Linux - you're probably right.

>> Those choices that you outlined appear to be different in terms whether
>> we solve the general problem and probably have some minor performance
>> impact or we solve the ad-hoc problem but make the system more
>> entangled. Here I'm more inclined to the first choice because this
>> particular scenario the performance impact should be negligible.
> 
> For the problem at hand there's no question about a
> performance effect. The question is whether doing this for _other_
> RCU callbacks would introduce a performance drop in certain cases.
> 

Yes, right. In that case this placement would mean we are going to lose
the partial context each time we take RCU in idle, is this correct? If
so that sounds like a common scenario to me and means there will be some
performance degradation, although I don't know how common it really is.

Anyway, if you're in favor of the previous approach I have no objections
as my understanding of Xen codebase is still partial.

Igor


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