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