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Date:   Thu, 21 May 2020 09:06:12 -0400
From:   Brian Gerst <brgerst@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.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 Wed, May 20, 2020 at 1:26 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> 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?

Yes, see boot_init_stack_canary().  That code looks a bit sketchy but
it's not wrong, for x86-64 at least.

It also doesn't seem to like "void *" compared to any other pointer type:

In function ‘fpregs_deactivate’,
    inlined from ‘fpu__drop’ at arch/x86/kernel/fpu/core.c:285:3:
./include/linux/compiler.h:379:38: error: call to
‘__compiletime_assert_317’ declared with attribute error: BUILD_BUG_ON
failed: !__same_type((fpu_fpregs_owner_ctx), ((void *)0))
  379 |  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
      |                                      ^
./include/linux/compiler.h:360:4: note: in definition of macro
‘__compiletime_assert’
  360 |    prefix ## suffix();    \
      |    ^~~~~~
./include/linux/compiler.h:379:2: note: in expansion of macro
‘_compiletime_assert’
  379 |  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
      |  ^~~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:39:37: note: in expansion of macro
‘compiletime_assert’
   39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
      |                                     ^~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:50:2: note: in expansion of macro ‘BUILD_BUG_ON_MSG’
   50 |  BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
      |  ^~~~~~~~~~~~~~~~
./arch/x86/include/asm/percpu.h:105:2: note: in expansion of macro
‘BUILD_BUG_ON’
  105 |  BUILD_BUG_ON(!__same_type(_var, _val));    \
      |  ^~~~~~~~~~~~
./arch/x86/include/asm/percpu.h:338:37: note: in expansion of macro
‘percpu_to_op’
  338 | #define this_cpu_write_8(pcp, val)  percpu_to_op(8, volatile,
"mov", (pcp), val)
      |                                     ^~~~~~~~~~~~
./include/linux/percpu-defs.h:380:11: note: in expansion of macro
‘this_cpu_write_8’
  380 |   case 8: stem##8(variable, __VA_ARGS__);break;  \
      |           ^~~~
./include/linux/percpu-defs.h:508:34: note: in expansion of macro
‘__pcpu_size_call’
  508 | #define this_cpu_write(pcp, val)
__pcpu_size_call(this_cpu_write_, pcp, val)
      |                                  ^~~~~~~~~~~~~~~~
./arch/x86/include/asm/fpu/internal.h:525:2: note: in expansion of
macro ‘this_cpu_write’
  525 |  this_cpu_write(fpu_fpregs_owner_ctx, NULL);
      |  ^~~~~~~~~~~~~~

>
> 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.

I think the original code is more to catch a mismatch between pointers
and integers.  It doesn't seem to care about truncation

> 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?

Not necessarily.  Some things like registers are naturally 32-bit on a
32-bit kernel and 64-bit on a 64-bit kernel, so 'long' is appropriate
there.

--
Brian Gerst

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