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SubjectRe: [RESEND][PATCH v3 09/17] x86/static_call: Add out-of-line static call implementation
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@redhat.com>
>[peterz: fixed trampoline, rewrote patching code]
>Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>---
> 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@googlegroups.com

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