Re: [PATCH 6/7] xen/guest_access: Consolidate guest access helpers in xen/guest_access.h

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 07/04/2020 09:14, Jan Beulich wrote:
> On 04.04.2020 15:10, Julien Grall wrote:
> > From: Julien Grall <jgrall@xxxxxxxxxx>
> >
> > Most of the helpers to access guest memory are implemented the same way
> > on Arm and x86. The only differences are:
> >      - guest_handle_{from, to}_param(): while on x86 XEN_GUEST_HANDLE()
> >        and XEN_GUEST_HANDLE_PARAM() are the same, they are not on Arm. It
> >        is still fine to use the Arm implementation on x86.
> >      - __clear_guest_offset(): Interestingly the prototype does not match
> >        between the x86 and Arm. However, the Arm one is bogus. So the x86
> >        implementation can be used.
> >      - guest_handle{,_subrange}_okay(): They are validly differing
> >        because Arm is only supporting auto-translated guest and therefore
> >        handles are always valid.
> While I'm fine in principle with such consolidation, I'm afraid I
> really need to ask for some historical background to be added
> here. It may very well be that there's a reason for the separation
> (likely to be found in the removed ia64 or ppc ports), which may
> then provide a hint at why future ports may want to have these
> separated. If such reasons exist, I'd prefer to avoid the back and
> forth between headers. What we could do in such a case is borrow
> Linux'es asm-generic/ concept, and move the "typical"
> implementation there. (And of course if there were no noticable
> reasons for the split, the change as it is would be fine in
> general; saying so without having looked at the details of it,
> yet).
Looking at the history, ia64 and ppc used to include a common header 
called xen/xencomm.h from asm/guest_access.h.
This has now disappeared with the removal of the two ports.

Regarding future arch, the fact arm and x86 gives me some confidence we 
are unlikely going to get a new ABI for an arch. Do you see any reason to?
Cheers,

--
Julien Grall