Re: [RESEND][PATCH v3 09/17] x86/static_call: Add out-of-line static call implementation

[Fangrui Song]

On 2020-03-24, Peter Zijlstra wrote:
> Add the x86 out-of-line static call implementation.  For each key, a
> permanent trampoline is created which is the destination for all static
> calls for the given key.  The trampoline has a direct jump which gets
> patched by static_call_update() when the destination function changes.
>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> [peterz: fixed trampoline, rewrote patching code]
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
> arch/x86/Kconfig                   |    1 +
> arch/x86/include/asm/static_call.h |   22 ++++++++++++++++++++++
> arch/x86/kernel/Makefile           |    1 +
> arch/x86/kernel/static_call.c      |   31 +++++++++++++++++++++++++++++++
> 4 files changed, 55 insertions(+)
> create mode 100644 arch/x86/include/asm/static_call.h
> create mode 100644 arch/x86/kernel/static_call.c
>
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -205,6 +205,7 @@ config X86
> 	select HAVE_FUNCTION_ARG_ACCESS_API
> 	select HAVE_STACKPROTECTOR		if CC_HAS_SANE_STACKPROTECTOR
> 	select HAVE_STACK_VALIDATION		if X86_64
> +	select HAVE_STATIC_CALL
> 	select HAVE_RSEQ
> 	select HAVE_SYSCALL_TRACEPOINTS
> 	select HAVE_UNSTABLE_SCHED_CLOCK
> --- /dev/null
> +++ b/arch/x86/include/asm/static_call.h
> @@ -0,0 +1,22 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_STATIC_CALL_H
> +#define _ASM_STATIC_CALL_H
> +
> +#include <asm/text-patching.h>
> +
> +/*
> + * For CONFIG_HAVE_STATIC_CALL, this is a permanent trampoline which
> + * does a direct jump to the function.  The direct jump gets patched by
> + * static_call_update().
> + */
> +#define ARCH_DEFINE_STATIC_CALL_TRAMP(name, func)			\
> +	asm(".pushsection .text, \"ax\"				\n"	\
> +	    ".align 4						\n"	\
> +	    ".globl " STATIC_CALL_TRAMP_STR(name) "		\n"	\
> +	    STATIC_CALL_TRAMP_STR(name) ":			\n"	\
> +	    "	jmp.d32 " #func "				\n"	\
> +	    ".type " STATIC_CALL_TRAMP_STR(name) ", @function	\n"	\
> +	    ".size " STATIC_CALL_TRAMP_STR(name) ", . - " STATIC_CALL_TRAMP_STR(name) " \n" \
> +	    ".popsection					\n")
> +
> +#endif /* _ASM_STATIC_CALL_H */

Hi Peter,

Coming here from https://github.com/ClangBuiltLinux/linux/issues/974

jmp.d32 is not recognized by clang integrated assembler.
The syntax appears to be very rarely used. According to Debian Code Search,
u-boot is the only project using this syntax.

In March 2017, gas added the pseudo prefix {disp32}
https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=86fa6981e7487e2c2df4337aa75ed2d93c32eaf2
which generalizes jmp.d32  ({disp32} jmp foo)

I wonder whether the instruction jmp.d32 can be replaced with the trick in
arch/x86/include/asm/jump_label.h for clang portability.

% grep -A2 'jmp.d32' arch/x86/include/asm/jump_label.h
        /* Equivalent to "jmp.d32 \target" */
        .byte           0xe9
        .long           \target - .Lstatic_jump_after_\@
...

+ clang-built-linux@xxxxxxxxxxxxxxxx