linux-kernel - Re: [PATCH v2 09/10] x86/percpu: Clean up percpu_stable

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Date:   Tue, 2 Jun 2020 10:19:51 -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 v2 09/10] x86/percpu: Clean up percpu_stable_op()

On Mon, Jun 1, 2020 at 4:43 PM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> On Sat, May 30, 2020 at 3:11 PM Brian Gerst <brgerst@...il.com> wrote:
> >
> > Use __pcpu_size_call_return() to simplify this_cpu_read_stable().
>
> Clever! As in this_cpu_read() in include/linux/percpu-defs.h.  Could
> be its own patch before this, but it's fine.
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
>
> > Also remove __bad_percpu_size() which is now unused.
> >
> > Signed-off-by: Brian Gerst <brgerst@...il.com>
> > ---
> >  arch/x86/include/asm/percpu.h | 41 ++++++++++-------------------------
> >  1 file changed, 12 insertions(+), 29 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
> > index 7efc0b5c4ff0..cf2b9c2a241e 100644
> > --- a/arch/x86/include/asm/percpu.h
> > +++ b/arch/x86/include/asm/percpu.h
> > @@ -85,7 +85,6 @@
> >
> >  /* For arch-specific code, we can use direct single-insn ops (they
> >   * don't give an lvalue though). */
> > -extern void __bad_percpu_size(void);
> >
> >  #define __pcpu_type_1 u8
> >  #define __pcpu_type_2 u16
> > @@ -167,33 +166,13 @@ do {                                                                      \
> >         (typeof(_var))(unsigned long) pfo_val__;                        \
> >  })
> >
> > -#define percpu_stable_op(op, var)                      \
> > -({                                                     \
> > -       typeof(var) pfo_ret__;                          \
> > -       switch (sizeof(var)) {                          \
> > -       case 1:                                         \
> > -               asm(op "b "__percpu_arg(P1)",%0"        \
>
> What does the `P` do here?
> https://gcc.gnu.org/onlinedocs/gcc/Simple-Constraints.html#Simple-Constraints
> says can be machine dependent integral literal in a certain range.
> https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html#Machine-Constraints
> doesn't document `P` for x86 though...

https://gcc.gnu.org/onlinedocs/gcc-10.1.0/gcc/Extended-Asm.html#x86-Operand-Modifiers

Removing the 'P' modifier results in this:
        movq %gs:$current_task, %rdx    #, pfo_val__

This is because the 'p' constraint treats a memory address as a
constant.  I tried replacing it with __this_cpu_read(), which since
commit 0b9ccc0a should have similar non-volatile semantics.  But the
compiler still reloaded it on every use, so I left the asm template
as-is for now until that can be resolved.

--
Brian Gerst