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Date:   Sun, 5 Apr 2020 18:08:59 -0700
From:   Fangrui Song <maskray@google.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org, rostedt@goodmis.org,
        mhiramat@kernel.org, bristot@redhat.com, jbaron@akamai.com,
        torvalds@linux-foundation.org, tglx@linutronix.de,
        mingo@kernel.org, namit@vmware.com, hpa@zytor.com, luto@kernel.org,
        ard.biesheuvel@linaro.org, jpoimboe@redhat.com,
        clang-built-linux@googlegroups.com
Subject: Re: [RESEND][PATCH v3 09/17] x86/static_call: Add out-of-line static
 call implementation
Message-ID: <20200406010859.bcfouhukcgmg2on7@google.com>
References: <20200324135603.483964896@infradead.org>
 <20200324142245.819003994@infradead.org>
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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