Re: [Xen-devel] Issues regarding "mem_access: Add helper API to setup ring and enable mem_access"

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 30.06.14 at 15:10, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 30/06/14 14:09, Ian Campbell wrote:
>> On Mon, 2014-06-30 at 14:07 +0100, Andrew Cooper wrote:
>>> On 30/06/14 13:44, Ian Campbell wrote:
>>>> On Mon, 2014-06-30 at 13:42 +0100, Ian Jackson wrote:
>>>>> Tamas Lengyel writes ("Re: [Xen-devel] Issues regarding "mem_access: Add 
> helper API to setup ring and enable mem_access""):
>>>>>>     > Now with this function being reintroduced, it becomes more 
>>>>>> complicated
>>>>>>     > to determine which version of the mem_access API does Xen actually
>>>>>>     > provide. A #define indicating mem_access API version would nicely
>>>>>>     > overcome this issue, or  naming xc_mem_event_enable something else.
>>>>>>
>>>>>>     Doesn't configure support checking for functions with a given 
>>>>>> prototype?
>>>>>>
>>>>>> It does but in a very hacky way, essentially trying to compile code 
>>>>>> where 
> the
>>>>>> function is being called with different prototypes. We can work around 
>>>>>> it 
> but a
>>>>>> clean solution would be preferred at some point.
>>>>> I agree with your criticism, TBH.  Aravindh/Ian, can we rename this
>>>>> function ?
>>>> I have no objection to some other name.
>>>>
>>>>>> This is now a rather opaque behavior of libxc. As it is not merged
>>>>>> into master I guess I will just submit a patch for it..
>>>>> Yes.  I think this would be the right approach to addressing both of
>>>>> these problems.  (Please send two patches, one for each.)
>>>>>
>>>>> Thanks,
>>>>> Ian.
>>> There is a bug in Xen.  domctl pause and unpause hypercalls are not
>>> properly refcounted.  Patch on its way.
>> I had convinced myself by reading that they were, but Tamas'
>> experimental evidence obviously counters that ;-)
> 
> The "d->is_paused_by_controller" is a boolean, not an atomic count.
> 
> The second pause doesn't add an extra pause ref to the domain, and the
> first unpause resumes the domain.

Which, as you see, is (in the hypervisor) intended behavior. If the
tool stack wants this to be ref-counted, it needs to do so by itself.

Jan


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