Re: [Xen-devel] [PATCH v3 1/2] Interface for grant copy operation in libs.

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On Wed, Jun 22, 2016 at 3:52 PM, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
>> I think the best solution is to allow the osdep code to provide the
>> implementation of xengnttab_grant_copy_segment_t, allowing the Linux
>> code to do:
>>
>> typedef ioctl_gntdev_grant_copy_segment xengnttab_grant_copy_segment_t
>>
>> You should still provide the generic structure as well, for those
>> platforms that don't provide their own optimized version.
>>
>
> We can't do that (yet). This means we open the door for divergence on
> different platforms.
>
> Basically this approach requires each platform to do the same thing
> (typedef) This implies any application that uses libxengnttab will need
> to test what platform it runs on. It is just pushing the issue somewhere
> else.
>
> Still, I think I would wait a bit for other people to weight in because
> I'm not sure if my concern is wrong headed.

I tend to be sympathetic to David's argument here.  The library has to
provide some ABI to callers; and it has to know the appropriate Linux
ABI in order to translate from the library ABI to the Linux ABI.  If
it happens to know these are the same, I don't see a reason not to
"translate" it by just by casting the pointer.

If we want to declare the library ABI in a stand-alone fashion (i.e.,
instead of just doing a typedef, so that the library definition is the
same on all platforms), then having some compile-time checking to make
sure that the layouts of the two structures are identical makes sense.
Beyond that, I'm not sure what the extra copying really buys us.

 -George

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