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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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 ;-)
>
> Ian.
>

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.

~Andrew

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