Re: [PATCH V7 13/20] riscv: compat: process: Add UXL_32 support in start_thread

[Arnd Bergmann <arnd@arndb.de>]

On Sat, Mar 12, 2022 at 3:13 AM Guo Ren <guoren@kernel.org> wrote:
> On Fri, Mar 11, 2022 at 9:38 PM Ben Dooks <ben.dooks@codethink.co.uk> wrote:
> > On 11/03/2022 02:38, Guo Ren wrote:
> > >> --- a/arch/riscv/kernel/process.c
> > >> +++ b/arch/riscv/kernel/process.c
> > >> @@ -97,6 +97,11 @@ void start_thread(struct pt_regs *regs, unsigned long pc,
> > >>          }
> > >>          regs->epc = pc;
> > >>          regs->sp = sp;
> > >> +
> > > FIxup:
> > >
> > > + #ifdef CONFIG_COMPAT
> > >> +       if (is_compat_task())
> > >> +               regs->status = (regs->status & ~SR_UXL) | SR_UXL_32;
> > >> +       else
> > >> +               regs->status = (regs->status & ~SR_UXL) | SR_UXL_64;
> > > + #endif
> > >
> > > We still need "#ifdef CONFIG_COMPAT" here, because for rv32 we can't
> > > set SR_UXL at all. SR_UXL is BIT[32, 33].
> >
> > would an if (IS_ENABLED(CONFIG_COMPAT)) { } around the lot be better
> > than an #ifdef here?
>
> I don't think, seems #ifdef CONFIG_COMPAT is more commonly used in arch/*

We used to require an #ifdef check around is_compat_task(), so there are
a lot of stale #ifdefs that could be removed. In general, 'if (IS_ENABLED())'
is considered more readable than #ifdef inside of a function. In this case
there are a number of better ways to write the function if you want to get
into the details:

 - firstly, you should remove the #ifdef check around the definition of
   SR_UXL, otherwise the IS_ENABLED() check does not work.

 - you can use an 'if (!IS_ENABLED(CONFIG_COMPAT)) \\ return;' ahead of the
   assignment since that is at the end of  the function.

 - you can remove the bit masking since 'regs->status' is initialized above it,
   adding in only the one bit, shortening it to

    if (IS_ENABLED(CONFIG_COMPAT))
               regs->status |= is_compat_task()) ? SR_UXL_32 : SR_UXL_64;

 - to make this more logical, I would suggest always assigning the SR_UXL
   bits regardless of CONFIG_COMPAT, and instead make it something like

  if (IS_ENABLED(CONFIG_32BIT) || is_compat_task())
            regs->status = | SR_UXL_32;
  else
            regs->status = | SR_UXL_64;

       Arnd