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


On 09/04/2020 07:30, Jan Beulich wrote:
On 09.04.2020 00:05, Julien Grall wrote:
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,

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?

Well, there surely had be a reason for ia64 and ppc to use a
different approach.

This was introduced way before my time in Xen. AFAICT, you were already part of the community when 'xencomm' were alive.

There are not much information about it only nor in the commits. So do you mind sharing a bit more what 'xencomm' meant to be and why it wasn't introduced on x86?

I don't see why a new port may not also want
to go that route instead of the x86/Arm one.
I could accept that someone would want to reinvent a new ABI from scratch for just an hypothetical new arch. However it would be quite an effort to reinvent XEN_GUEST_HANDLE(). The chance is RISC-V is only going to re-use what Arm did as Arm already did with x86.

I would like to avoid to introduce a new directory asm-generic with just one header in it. Maybe you have some other headers in mind?


Julien Grall



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