Re: [Xen-devel] [PATCH 07/12] xen/arm: cpuerrata: Match register size with value size in check_workaround_*

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 17 Apr 2019, Julien Grall wrote:
> Hi,
> 
> On 4/17/19 9:28 PM, Stefano Stabellini wrote:
> > On Wed, 27 Mar 2019, Julien Grall wrote:
> > > Clang is pickier than GCC for the register size in asm statement. It
> > > expects the register size to match the value size.
> > > 
> > > The asm statement expects a 32-bit (resp. 64-bit) value on Arm32
> > > (resp. Arm64) whereas the value is a boolean (Clang consider to be
> > > 32-bit).
> > > 
> > > It would be possible to impose 32-bit register for both architecture
> > > but this require the code to use __OP32. However, it does not really
> > > improve the assembly generated. Instead, replace switch the variable
> > > to use register_t.
> > > 
> > > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> > > ---
> > >   xen/include/asm-arm/cpuerrata.h | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/xen/include/asm-arm/cpuerrata.h
> > > b/xen/include/asm-arm/cpuerrata.h
> > > index 55ddfda272..88ef3ca934 100644
> > > --- a/xen/include/asm-arm/cpuerrata.h
> > > +++ b/xen/include/asm-arm/cpuerrata.h
> > > @@ -14,7 +14,7 @@ static inline bool check_workaround_##erratum(void)
> > > \
> > >           return false;                                           \
> > >       else                                                        \
> > >       {                                                           \
> > > -        bool ret;                                               \
> > > +        register_t ret;                                         \
> > >                                                                   \
> > >           asm volatile (ALTERNATIVE("mov %0, #0",                 \
> > >                                     "mov %0, #1",                 \
> > 
> > This is OK. Could you please also change the return statement below?
> > Maybe something like:
> > 
> >    return unlikely(!!ret);
> Why? The compiler will implicitly convert the int to bool. 0 will turn to
> false, all the other will be true.
> 
> We actually been actively removing !! when the type is bool (see the example
> in get_paged_frame in common/grant_table.c).

Really? Too bad, I loved the explicit conversions to bool. This is a
matter of code style, not correctness, so usually I wouldn't care much.
But I went to read MISRA-C to figure out if there are any differences
from that point of view. From Rule 10.3, it looks like it is not
compliant, because they say that:

  bool_t bla = 0;

is not MISRA-C compliant. While:

  int c = 1;
  bool_t bla = c == 0;

is compliant. So, if I read this right:

  return !!ret //compliant
  return ret;  //not compliant

I am not 100% sure though.

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