Re: [PATCH 10/15] static_call: Add basic static call infrastructure

[Nadav Amit]

> On Jun 5, 2019, at 6:08 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> 
> 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%7C3f2ebbeff15e444d2fa008d6e9b9023f%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636953378182765229&amp;sdata=WHceTWVt%2BNu1RFBv8jHp2Tw7VZuI5HxvHt%2FrWnjAmm4%3D&amp;reserved=0
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> 
> 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.
> 
> Cc: x86@xxxxxxxxxx
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Julia Cartwright <julia@xxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> Cc: Jason Baron <jbaron@xxxxxxxxxx>
> Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Jiri Kosina <jkosina@xxxxxxx>
> Cc: Edward Cree <ecree@xxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: David Laight <David.Laight@xxxxxxxxxx>
> Cc: Jessica Yu <jeyu@xxxxxxxxxx>
> Cc: Nadav Amit <namit@xxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Link: https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flkml.kernel.org%2Fr%2Fa01f733889ebf4bc447507ab8041a60378eaa89f.1547073843.git.jpoimboe%40redhat.com&amp;data=02%7C01%7Cnamit%40vmware.com%7C3f2ebbeff15e444d2fa008d6e9b9023f%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636953378182765229&amp;sdata=n5wgu%2FxNZiG77ExBcoT2wo7ak9xqyfJH3H8SMyxZj38%3D&amp;reserved=0
> ---
> arch/Kconfig                      |    3 
> include/linux/static_call.h       |  135 ++++++++++++++++++++++++++++++++++++++
> include/linux/static_call_types.h |   13 +++
> 3 files changed, 151 insertions(+)
> create mode 100644 include/linux/static_call.h
> create mode 100644 include/linux/static_call_types.h
> 
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -927,6 +927,9 @@ config LOCK_EVENT_COUNTS
> 	  the chance of application behavior change because of timing
> 	  differences. The counts are reported via debugfs.
> 
> +config HAVE_STATIC_CALL
> +	bool
> +
> source "kernel/gcov/Kconfig"
> 
> source "scripts/gcc-plugins/Kconfig"
> --- /dev/null
> +++ b/include/linux/static_call.h
> @@ -0,0 +1,135 @@
> +/* 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(key, func);
> + *   DEFINE_STATIC_CALL(key, func);
> + *   static_call(key, args...);
> + *   static_call_update(key, 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_key' reference, associated with func_a() by default
> + *   DEFINE_STATIC_CALL(my_key, func_a);
> + *
> + *   # Call func_a()
> + *   static_call(my_key, arg1, arg2);
> + *
> + *   # Update 'my_key' to point to func_b()
> + *   static_call_update(my_key, func_b);
> + *
> + *   # Call func_b()
> + *   static_call(my_key, arg1, arg2);

I think that this calling interface is not very intuitive. I understand that
the macros/objtool cannot allow the calling interface to be completely
transparent (as compiler plugin could). But, can the macros be used to
paste the key with the âstatic_callâ? I think that having something like:

  static_call__func(arg1, arg2)

Is more readable than

  static_call(func, arg1, arg2)

> +}
> +
> +#define static_call_update(key, func)					\
> +({									\
> +	BUILD_BUG_ON(!__same_type(func, STATIC_CALL_TRAMP(key)));	\
> +	__static_call_update(&key, func);				\
> +})

Is this safe against concurrent module removal?