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

[Julien Grall <julien.grall@xxxxxxx>]

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...
Cheers,

--
Julien Grall

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