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Re: [Xen-devel] [RFC Patch v4 7/9] correct xc_domain_save()'s return value

On 09/23/2014 05:00 PM, Andrew Cooper wrote:
> On 23/09/14 03:14, Wen Congyang wrote:
>> On 09/22/2014 10:06 PM, Andrew Cooper wrote:
>>> On 22/09/14 14:13, Ian Campbell wrote:
>>>> On Mon, 2014-09-22 at 14:03 +0100, Ian Jackson wrote:
>>>>> Ian Campbell writes ("Re: [Xen-devel] [RFC Patch v4 7/9] correct 
>>>>> xc_domain_save()'s return value"):
>>>>>> libxc doesn't know that, if it is important then it seems like the
>>>>>> failure + errno ought to be marshalled across the IPC link.
>>>>> Yes, but ...
>>>>>> It may be that this can be easily handled in
>>>>>> libxl__srm_callout_sendreply + helper_getreply. Ian J -- what do you
>>>>>> think?
>>>>> ... while that would be possbile, we have another option.
>>>>> We could say that the callbacks return errno values.  That would
>>>>> simplify the API and avoid having the IPC involve accesses to global
>>>>> variables (ie, things not in the functions' parameter lists).
>>>>> If we do that then it becomes the responsibility of xc_domain_save to
>>>>> either change its own API to return errno, or to save the callback's
>>>>> return value in errno.
>>>> Hrm. libxc is already a complete mess wrt error returning/handling
>>>> because some proportion of the code incorrectly does/assumes this sort
>>>> of thing is happening (because people were confused about the syscall
>>>> returns from the kernel vs. process context). Having a place in libxc
>>>> where this is now done on purpose seems a bit like setting the rope on
>>>> fire to me...
>>>> Ian.
>>> libxc is an absolute mess, but this is far from the only codepath (even
>>> in xc_domain_save()) which ends up like this.
>>> The *only* safe assumption is that ==0 is success and !=0 is failure for
>>> xc_domain_save(), and errno may or may not be relevant, whether rc is -1
>>> or not.
>> Do you mean: errno is undefined even if rc is -1?
>> Thanks
>> Wen Congyang
> Correct, last time I checked.  The error handling in libxc is in dire
> need of fixing from scratch.

For v2 version migration, we can do it like this. But xc_domain_save()
is a public API, and is used some years. So I am not sure if I can
change its behavior...

Wen Congyang

> ~Andrew
> .

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