linux-kernel - Re: [PATCH 2/7] x86/percpu: Clean up percpu_to

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Message-ID: <CAKwvOdmwh92BP2=-d4Apr07i6uKb21M15MAz-v4S5RfQ16-fPA@mail.gmail.com>
Date:   Wed, 20 May 2020 10:26:13 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Brian Gerst <brgerst@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/7] x86/percpu: Clean up percpu_to_op()

On Mon, May 18, 2020 at 8:38 PM Brian Gerst <brgerst@...il.com> wrote:
>
> On Mon, May 18, 2020 at 5:15 PM Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
> >
> > On Sun, May 17, 2020 at 8:29 AM Brian Gerst <brgerst@...il.com> wrote:
> > >
> > > The core percpu macros already have a switch on the data size, so the switch
> > > in the x86 code is redundant and produces more dead code.
> > >
> > > Also use appropriate types for the width of the instructions.  This avoids
> > > errors when compiling with Clang.
> > >
> > > Signed-off-by: Brian Gerst <brgerst@...il.com>
> > > ---
> > >  arch/x86/include/asm/percpu.h | 90 ++++++++++++++---------------------
> > >  1 file changed, 35 insertions(+), 55 deletions(-)
> > >
> > > diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
> > > index 89f918a3e99b..233c7a78d1a6 100644
> > > --- a/arch/x86/include/asm/percpu.h
> > > +++ b/arch/x86/include/asm/percpu.h
> > > @@ -117,37 +117,17 @@ extern void __bad_percpu_size(void);
> > >  #define __pcpu_reg_imm_4(x) "ri" (x)
> > >  #define __pcpu_reg_imm_8(x) "re" (x)
> > >
> > > -#define percpu_to_op(qual, op, var, val)               \
> > > -do {                                                   \
> > > -       typedef typeof(var) pto_T__;                    \
> > > -       if (0) {                                        \
> > > -               pto_T__ pto_tmp__;                      \
> > > -               pto_tmp__ = (val);                      \
> > > -               (void)pto_tmp__;                        \
> > > -       }                                               \
> > > -       switch (sizeof(var)) {                          \
> > > -       case 1:                                         \
> > > -               asm qual (op "b %1,"__percpu_arg(0)     \
> > > -                   : "+m" (var)                        \
> > > -                   : "qi" ((pto_T__)(val)));           \
> > > -               break;                                  \
> > > -       case 2:                                         \
> > > -               asm qual (op "w %1,"__percpu_arg(0)     \
> > > -                   : "+m" (var)                        \
> > > -                   : "ri" ((pto_T__)(val)));           \
> > > -               break;                                  \
> > > -       case 4:                                         \
> > > -               asm qual (op "l %1,"__percpu_arg(0)     \
> > > -                   : "+m" (var)                        \
> > > -                   : "ri" ((pto_T__)(val)));           \
> > > -               break;                                  \
> > > -       case 8:                                         \
> > > -               asm qual (op "q %1,"__percpu_arg(0)     \
> > > -                   : "+m" (var)                        \
> > > -                   : "re" ((pto_T__)(val)));           \
> > > -               break;                                  \
> > > -       default: __bad_percpu_size();                   \
> > > -       }                                               \
> > > +#define percpu_to_op(size, qual, op, _var, _val)                       \
> > > +do {                                                                   \
> > > +       __pcpu_type_##size pto_val__ = __pcpu_cast_##size(_val);        \
> > > +       if (0) {                                                        \
> > > +               typeof(_var) pto_tmp__;                                 \
> > > +               pto_tmp__ = (_val);                                     \
> > > +               (void)pto_tmp__;                                        \
> > > +       }                                                               \
> >
> > Please replace the whole `if (0)` block with:
> > ```c
> > __same_type(_var, _val);
> > ```
> > from include/linux/compiler.h.
>
> The problem with __builtin_types_compatible_p() is that it considers
> unsigned long and u64 (aka unsigned long long) as different types even
> though they are the same width on x86-64.  While this may be a good
> cleanup to look at in the future, it's not a simple drop-in
> replacement.

Does it trigger errors in this case?

It's interesting to know how this trick differs from
__builtin_types_compatible_p().  Might even be helpful to wrap this
pattern in a macro with a comment with the pros/cons of this approach
vs __same_type.

On the other hand, the use of `long` seems tricky in x86 code as x86
(32b) is ILP32 but x86_64 (64b) is LP64.  So the use of `long` is
ambiguous in the sense that it's a different size depending on the
target ABI.  Wouldn't it potentially be a bug for x86 kernel code to
use `long` percpu variables (or rather mix `long` and `long long` in
the same operation) in that case, since the sizes of the two would be
different for i386?
-- 
Thanks,
~Nick Desaulniers