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    Subject[PATCH v3 23/23] x86_64: use relative pointers with dynamic debug
    Date
    Similar to how x86_64 uses bug_entry-relative pointers to reduce
    sizeof(struct bug_entry), the same thing can now be done for struct
    _ddebug, saving 16 bytes for each of those (i.e., each pr_debug, dev_dbg
    etc. in a CONFIG_DYNAMIC_DEBUG kernel).

    Note the use of .ifndef/.endif in asm to avoid

    fs/aio.c:1382: Error: symbol `__UNIQUE_ID_ddebug112' is already defined

    This is due to uses of pr_debug et al in functions that get inlined. In
    such a case, __COUNTER__ is obviously only expanded once, but the asm()
    is repeated once for every inlined instance. Letting all instances share
    the same descriptor object is the right thing to do; that is also what
    happens when it is the compiler that defines a static object inside an
    inline(d) function.

    That, however, leads to another problem if the same identifier is used
    in two different DEFINE_DYNAMIC_DEBUG_METADATA() in the same translation
    unit: They would both end up referring to the same (first occuring)
    instance. So the second one would end up using the wrong file, line,
    function etc. information. All current in-tree macro users of
    DEFINE_DYNAMIC_DEBUG_METADATA() have been updated to ensure they use
    UNIQUE_ID to generate the identifier.

    To prevent such a problem from going unnoticed, we use a guard symbol in
    assembly whose value is unique per DEFINE_DYNAMIC_DEBUG_METADATA
    invocation.

    Cc: x86@kernel.org
    Cc: Ingo Molnar <mingo@kernel.org>
    Cc: Jason Baron <jbaron@akamai.com>
    Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
    ---
    arch/x86/Kconfig | 1 +
    arch/x86/include/asm/dynamic_debug.h | 58 ++++++++++++++++++++++++++++
    arch/x86/kernel/macros.S | 1 +
    3 files changed, 60 insertions(+)
    create mode 100644 arch/x86/include/asm/dynamic_debug.h

    diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
    index ba7e3464ee92..c7cb31f39e5a 100644
    --- a/arch/x86/Kconfig
    +++ b/arch/x86/Kconfig
    @@ -91,6 +91,7 @@ config X86
    select CLOCKSOURCE_WATCHDOG
    select DCACHE_WORD_ACCESS
    select DMA_DIRECT_OPS
    + select DYNAMIC_DEBUG_RELATIVE_POINTERS if X86_64
    select EDAC_ATOMIC_SCRUB
    select EDAC_SUPPORT
    select GENERIC_CLOCKEVENTS
    diff --git a/arch/x86/include/asm/dynamic_debug.h b/arch/x86/include/asm/dynamic_debug.h
    new file mode 100644
    index 000000000000..80730409183f
    --- /dev/null
    +++ b/arch/x86/include/asm/dynamic_debug.h
    @@ -0,0 +1,58 @@
    +/* SPDX-License-Identifier: GPL-2.0 */
    +#ifndef _ASM_X86_DYNAMIC_DEBUG_H
    +#define _ASM_X86_DYNAMIC_DEBUG_H
    +
    +#ifndef __ASSEMBLY__
    +
    +#ifdef DEBUG
    +#define _DPRINTK_INIT_TRUE 1
    +#else
    +#define _DPRINTK_INIT_TRUE 0
    +#endif
    +
    +#define DEFINE_DYNAMIC_DEBUG_METADATA(name, fmt) \
    + extern struct _ddebug name; \
    + asm volatile("DYNAMIC_DEBUG_METADATA" \
    + " ident="__stringify(name) \
    + " modname=%c0" \
    + " func=%c1" \
    + " file=%c2" \
    + " fmt=%c3" \
    + " flag_line=%c4" \
    + " init_true=%c5" \
    + " cnt=%c6" \
    + : : "i" (KBUILD_MODNAME), "i" (__func__), \
    + "i" (__FILE__), "i" (fmt), \
    + "i" (_DPRINTK_FLAGS_LINENO_INIT), \
    + "i" (_DPRINTK_INIT_TRUE), \
    + "i" (__COUNTER__) \
    + )
    +
    +#else /* __ASSEMBLY__ */
    +
    +.macro DYNAMIC_DEBUG_METADATA ident modname func file fmt flag_line init_true cnt
    +.ifndef \ident
    + .pushsection __verbose,"aw"
    +\ident :
    + .long \modname - \ident
    + .long \func - \ident
    + .long \file - \ident
    + .long \fmt - \ident
    + .long \flag_line
    + .long 0
    + .if \init_true
    + STATIC_KEY_INIT_TRUE
    + .else
    + STATIC_KEY_INIT_FALSE
    + .endif
    + .popsection
    + .set \ident\().ddebug.once, \cnt
    +.elseif \ident\().ddebug.once - \cnt
    + .error "'\ident' used as _ddebug identifier more than once"
    +.endif
    +.endm
    +
    +#endif /* __ASSEMBLY__ */
    +
    +#endif /* _ASM_X86_DYNAMIC_DEBUG_H */
    +
    diff --git a/arch/x86/kernel/macros.S b/arch/x86/kernel/macros.S
    index 161c95059044..2c2b8e5229a9 100644
    --- a/arch/x86/kernel/macros.S
    +++ b/arch/x86/kernel/macros.S
    @@ -14,3 +14,4 @@
    #include <asm/asm.h>
    #include <asm/cpufeature.h>
    #include <asm/jump_label.h>
    +#include <asm/dynamic_debug.h>
    --
    2.19.1.6.gbde171bbf5
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