Re: [Xen-devel] [PATCH 05/19] xen: arm: Add and use r/o+raz and w/o+wi helpers

[Julien Grall <julien.grall.oss@xxxxxxxxx>]

Hi Ian,

On 16/04/2015 17:22, Ian Campbell wrote:
> On Fri, 2015-04-03 at 14:51 +0200, Julien Grall wrote:
> > On 31/03/2015 12:07, Ian Campbell wrote:
> > > Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > > ---
> > >    xen/arch/arm/traps.c |   52 
> > > ++++++++++++++++++++++++++++++++------------------
> > >    1 file changed, 33 insertions(+), 19 deletions(-)
> > >
> > > diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
> > > index 8b1846a..ebc09f9 100644
> > > --- a/xen/arch/arm/traps.c
> > > +++ b/xen/arch/arm/traps.c
> > > @@ -1587,6 +1587,34 @@ static void handle_raz_wi(struct cpu_user_regs *regs,
> > >        advance_pc(regs, hsr);
> > >    }
> > >
> > > +/* Write only + write ignore */
> > > +static void handle_wo_wi(struct cpu_user_regs *regs,
> > > +                         register_t *reg,
> > This helper handles WO and WI which doesn't require to modify the register.
> >
> > I would pass a register_t rather than register_t* in order to make clear
> > that the register of the guest won't change.
> I deliberately made handle_* all have the same prototype for
> consistency.
Ok.


> > > +/* Read only + read as zero */
> > This comment may confuse developer who wants to implement RO register
> > which another value than 0.
> >
> > I got confuse too. It would be nice to expand the comment for the RO case.
> I'm afraid I don't understand the confusion so I'm not sure how to
> clarify. What did you think this comment was saying?
When I read the comment I though you were implemented two distinct part:
RO and RAZ.

As the value return by RO may not always be 0 (we have a handful of 
cases in traps.c), I didn't understand why you were implementing both 
within the same helper.
Although this helper choose to implement RO as RAZ. So I think it would 
be good to mention it in order to avoid confusion later.
Regards,

--
Julien Grall

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