Re: [Xen-devel] [PATCH 1/6] xen/arm: smccc-1.1: Make return values unsigned long

[Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>]

Julien,

On 27.08.18 18:23, Julien Grall wrote:
> On 27/08/2018 15:03, Volodymyr Babchuk wrote:
> > Hi  Julien, Marc
> Hi,
>
> > On 24.08.18 19:58, Julien Grall wrote:
> > > From: Marc Zyngier <marc.zyngier@xxxxxxx>
> > >
> > > An unfortunate consequence of having a strong typing for the input
> > > values to the SMC call is that it also affects the type of the
> > > return values, limiting r0 to 32 bits and r{1,2,3} to whatever
> > > was passed as an input. >
> > > Let's turn everything into "unsigned long", which satisfies the
> > > requirements of both architectures, and allows for the full
> > > range of return values.
> > Maybe it better to use register_t then? By definition register_t has 
> > the same size as a CPU register and SMC uses CPU registers to pass 
> > parameters/return values.
> The code is based on Linux series (posted last Friday). I don't much 
> want to diverge too much from it. So unsigned "unsigned long" here is ok.
> > > Reported-by: Stefano Stabellini <stefanos@xxxxxxxxxx>
> > > Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
> > > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> > > ---
> > >   xen/include/asm-arm/smccc.h | 22 +++++++++++-----------
> > >   1 file changed, 11 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/xen/include/asm-arm/smccc.h b/xen/include/asm-arm/smccc.h
> > > index 74c13f8419..a31d67a1de 100644
> > > --- a/xen/include/asm-arm/smccc.h
> > > +++ b/xen/include/asm-arm/smccc.h
> > > @@ -119,35 +119,35 @@ struct arm_smccc_res {
> > >   #define __declare_arg_0(a0, res)                        \
> > >       struct arm_smccc_res    *___res = res;              \
> > > -    register uin32_t        r0 asm("r0") = a0;          \
> > > +    register unsigned long  r0 asm("r0") = (uint32_t)a0;\
> > Do you really want to silently drop upper 32 bits of the argument?
> > I know, that SMCCC states that function id is a 32-bit value,
> > but I don't think that it is a good place to enforce this
> > behavior.
> > I think it is better to allow user to shoot in his leg in this case.
> I don't see any issue with casting here. This is what the SMCCC request 
> for x0 and after all you silently drop upper 32-bit when using a static 
> inline function...
Yes, you are right.

Reviewed-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>

--
Volodymyr Babchuk

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