Re: [PATCH v2 09/10] x86/percpu: Clean up percpu_stable_op()

From: Brian Gerst
Date: Tue Jun 02 2020 - 10:20:05 EST


On Mon, Jun 1, 2020 at 4:43 PM Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
>
> On Sat, May 30, 2020 at 3:11 PM Brian Gerst <brgerst@xxxxxxxxx> 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@xxxxxxxxxx>
>
> > Also remove __bad_percpu_size() which is now unused.
> >
> > Signed-off-by: Brian Gerst <brgerst@xxxxxxxxx>
> > ---
> > 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