Re: [RESEND][PATCH v3 06/17] static_call: Add basic static call infrastructure

[Nadav Amit <namit@vmware.com>]

> On Mar 24, 2020, at 6:56 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> 
> Static calls are a replacement for global function pointers.  They use
> code patching to allow direct calls to be used instead of indirect
> calls.  They give the flexibility of function pointers, but with
> improved performance.  This is especially important for cases where
> retpolines would otherwise be used, as retpolines can significantly
> impact performance.
> 
> The concept and code are an extension of previous work done by Ard
> Biesheuvel and Steven Rostedt:
> 
>  https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flkml.kernel.org%2Fr%2F20181005081333.15018-1-ard.biesheuvel%40linaro.org&amp;data=02%7C01%7Cnamit%40vmware.com%7C48cb49e3bd65479a440008d7cfff207d%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C637206567005697724&amp;sdata=jLAdeYFX8C67QFeYtiguq9BfeN8zxjSa8gmtY%2F2nJQ8%3D&amp;reserved=0
>  https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flkml.kernel.org%2Fr%2F20181006015110.653946300%40goodmis.org&amp;data=02%7C01%7Cnamit%40vmware.com%7C48cb49e3bd65479a440008d7cfff207d%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C637206567005697724&amp;sdata=KBVx7EJo4EgzKaCccAaMP7C4L%2BYlQ6liO4wO7vv27MU%3D&amp;reserved=0
> 
> There are two implementations, depending on arch support:
> 
> 1) out-of-line: patched trampolines (CONFIG_HAVE_STATIC_CALL)
> 2) basic function pointers
> 
> For more details, see the comments in include/linux/static_call.h.
> 
> [peterz: simplified interface]
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> arch/Kconfig                      |    3 
> include/linux/static_call.h       |  137 ++++++++++++++++++++++++++++++++++++++
> include/linux/static_call_types.h |   15 ++++
> 3 files changed, 155 insertions(+)
> create mode 100644 include/linux/static_call.h
> create mode 100644 include/linux/static_call_types.h
> 
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -941,6 +941,9 @@ config HAVE_SPARSE_SYSCALL_NR
> 	  entries at 4000, 5000 and 6000 locations. This option turns on syscall
> 	  related optimizations for a given architecture.
> 
> +config HAVE_STATIC_CALL
> +	bool
> +
> source "kernel/gcov/Kconfig"
> 
> source "scripts/gcc-plugins/Kconfig"
> --- /dev/null
> +++ b/include/linux/static_call.h
> @@ -0,0 +1,137 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_STATIC_CALL_H
> +#define _LINUX_STATIC_CALL_H
> +
> +/*
> + * Static call support
> + *
> + * Static calls use code patching to hard-code function pointers into direct
> + * branch instructions.  They give the flexibility of function pointers, but
> + * with improved performance.  This is especially important for cases where
> + * retpolines would otherwise be used, as retpolines can significantly impact
> + * performance.
> + *
> + *
> + * API overview:
> + *
> + *   DECLARE_STATIC_CALL(name, func);
> + *   DEFINE_STATIC_CALL(name, func);
> + *   static_call(name)(args...);
> + *   static_call_update(name, func);
> + *
> + * Usage example:
> + *
> + *   # Start with the following functions (with identical prototypes):
> + *   int func_a(int arg1, int arg2);
> + *   int func_b(int arg1, int arg2);
> + *
> + *   # Define a 'my_name' reference, associated with func_a() by default
> + *   DEFINE_STATIC_CALL(my_name, func_a);

Do you want to support optional function attributes, such as “pure” and
“const”?