Re: [Xen-devel] [PATCH for-4.10 1/2] tools/libxc: Fix precopy_policy() to not pass a structure by value

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 16/10/17 14:40, Wei Liu wrote:
> On Fri, Oct 13, 2017 at 06:32:18PM +0100, Andrew Cooper wrote:
>> c/s 4d69b3495 "Introduce migration precopy policy" uses bogus reasoning to
>> justify passing precopy_stats by value.
>>
>> Under no circumstances can the precopy callback ever be executing in a
>> separate address space.
>>
> The callback is not executed in a separate address space. 
>
> Have you checked
> <1506365735-133776-4-git-send-email-Jennifer.Herbert@xxxxxxxxxx>?
>
> The open source toolstack spawns another process to save vm image. In
> order to let libxl control the process (in the future) there is
> information passed across process boundary.
>
> Your code might work for now because Joshua's patch is not yet applied.

I'm perfectly aware of that discussion, and it is factually incorrect. 
Nothing, not even Joshua's patch, can cause the callback to be executed
in a separate address space.

With Joshua's patch in place, the implementer of this callback is the
code generated by libxl_save_msgs_gen.pl, which is the aformentioned
extra process.  Passing by pointer or value has nothing to do with the
fact that the automatically generated code needs to know how to
serialise/deserialise the data to feed it back to the main process.

~Andrew

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