Re: [Xen-devel] [PATCH v4 12/16] xen/mm: Switch common/memory.c to use typesafe MFN

[Julien Grall <julien.grall@xxxxxxx>]

Hi Jan,

On 03/12/2018 06:39 AM, Jan Beulich wrote:
> > > > Julien Grall <julien.grall@xxxxxxx> 03/11/18 8:44 PM >>>
> > On 03/09/2018 05:33 PM, Wei Liu wrote:
> > > On Mon, Mar 05, 2018 at 07:41:54AM -0700, Jan Beulich wrote:
> > > > > > > On 05.03.18 at 15:18, <julien.grall@xxxxxxx> wrote:
> > > > > Also, do you have an opinion on Wei's suggestion:
> > > > >
> > > > > "What I meant was to make copy_{to,from}_guest* type-safe. I just feel it
> > > > > a bit strange you only created a wrapper for this file. I wonder why.
> > > > >
> > > > > Note I'm just asking question. That's not necessarily a good idea to
> > > > > turn them all in the end."
> > > > Well, I didn't really understand what he's after (in the context of
> > > > this series) - copy_{to,from}_guest() don't take or return MFNs or
> > > > GFNs.
> > > Fundamentally Julien's patch is to wrap around an existing API for this
> > > one file only. Why is this file special? Why not just make that class of
> > > APIs do what he wants?
> > >
> > > But that is going to be intrusive and a bit counter-intuitive.
> > I have quickly looked at it. The major problem I can see is it is not
> > possible to generically define for any typesafe. Indeed, TYPE_SAFE(...)
> > cannot define new macro and, AFAICT, it is not feasible to define static
> > inline for copy_* helpers.
> >
> > So we would need to introduce macros for each typesafe by hand. I can
> > move copy_mfn_to_guest in xen/mm.h if people think it could be useful.
> First of all - how often do we copy in/out individual MFNs? Not in many places,
> I think. Hence I'm afraid I continue to not see the value of such a construct,
> especially not as a wider-than-file-scope one.
I think you are right. Looking at the interface, we tend copy copy more 
often a GFN. We might want to provide an helper for typesafe GFN in the 
future.
Cheers.

--
Julien Grall

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