Re: [XEN PATCH v5 7/8] x86/mm: add defensive return

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Aug 29, 2024 at 09:04:37AM +0200, Jan Beulich wrote:
> On 29.08.2024 02:35, Stefano Stabellini wrote:
> > On Mon, 29 Jul 2024, Stefano Stabellini wrote:
> >> On Mon, 29 Jul 2024, Federico Serafini wrote:
> >>> Add defensive return statement at the end of an unreachable
> >>> default case. Other than improve safety, this meets the requirements
> >>> to deviate a violation of MISRA C Rule 16.3: "An unconditional `break'
> >>> statement shall terminate every switch-clause".
> >>>
> >>> Signed-off-by: Federico Serafini <federico.serafini@xxxxxxxxxxx>
> >>
> >> Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> >>
> >>> ---
> >>> No changes from v3 and v4, further feedback on this thread would be 
> >>> appreciated:
> >>> https://lists.xenproject.org/archives/html/xen-devel/2024-07/msg00474.html
> > 
> > Looking at the older threads, I looks like Jan suggested EACCES, I also
> > think it is marginally better. My R-b stands also for EACCES. Jan, I
> > think you should go ahead and fix on commit
> 
> No, I very definitely want a 2nd x86 maintainer opinion here. Or a better
> suggestion for the error code to use by anyone. After all, as you confirm,
> EACCES is only marginally better.

Hm, the only alternative I could suggest is using ERANGE, to signal
the value of opt_mmio_relax is out of the expected range, however that
could be confusing for the callers?

One benefit of using ERANGE is that there's currently no return path
in get_page_from_l1e() with that error code, so it would be very easy
to spot when an unexpected value of opt_mmio_relax is found.  However
there are no guarantees that further error paths might use that error
code.

Thanks, Roger.