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From: Mark Rutland <mark.rutland@arm.com>
To: Torsten Duwe <duwe@suse.de>
Cc: linux-arm-kernel@lists.infradead.org,
	Jessica Yu <jeyu@kernel.org>, Helge Deller <deller@gmx.de>,
	"James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
	linux-kernel@vger.kernel.org, amit.kachhap@arm.com,
	catalin.marinas@arm.com, james.morse@arm.com,
	jpoimboe@redhat.com, jthierry@redhat.com,
	linux-parisc@vger.kernel.org, mingo@redhat.com,
	peterz@infradead.org, rostedt@goodmis.org, svens@stackframe.org,
	takahiro.akashi@linaro.org, will@kernel.org
Subject: Re: [PATCHv2 2/8] module/ftrace: handle patchable-function-entry
Date: Thu, 31 Oct 2019 09:02:32 +0000
Message-ID: <20191031090231.GA3340@blommer> (raw)
In-Reply-To: <20191030150302.GA965@suse.de>

On Wed, Oct 30, 2019 at 04:03:02PM +0100, Torsten Duwe wrote:
> On Tue, Oct 29, 2019 at 04:58:26PM +0000, Mark Rutland wrote:
> > When using patchable-function-entry, the compiler will record the
> > callsites into a section named "__patchable_function_entries" rather
> > than "__mcount_loc". Let's abstract this difference behind a new
> > FTRACE_CALLSITE_SECTION, so that architectures don't have to handle this
> > explicitly (e.g. with custom module linker scripts).
> > 
> > As parisc currently handles this explicitly, it is fixed up accordingly,
> > with its custom linker script removed. Since FTRACE_CALLSITE_SECTION is
> > only defined when DYNAMIC_FTRACE is selected, the parisc module loading
> > code is updated to only use the definition in that case. When
> > DYNAMIC_FTRACE is not selected, modules shouldn't have this section, so
> > this removes some redundant work in that case.
> > 
> > I built parisc generic-{32,64}bit_defconfig with DYNAMIC_FTRACE enabled,
> > and verified that the section made it into the .ko files for modules.
> 
> This is because of remaining #ifdeffery in include/asm-generic/vmlinux.lds.h:
> 
> #ifdef CC_USING_PATCHABLE_FUNCTION_ENTRY
> #define MCOUNT_REC()    . = ALIGN(8);                           \
>                         __start_mcount_loc = .;                 \
>                         KEEP(*(__patchable_function_entries))   \
>                         __stop_mcount_loc = .;
> #else
> #define MCOUNT_REC()    . = ALIGN(8);                           \
>                         __start_mcount_loc = .;                 \
>                         KEEP(*(__mcount_loc))                   \
>                         __stop_mcount_loc = .;
> #endif

For modules we use a combination of scripts/module-common.lds and an
architecture's own module.lds, not vmlinux.lds.h. So I don't think the above is
relevant for modules.

For modules the kernel's ELF loader looks for the ELF ection, not the
__{start,stop}_mcount_loc symbols that we use for the main kernel image.

FWIW, when building a module, I see the following linker operations:

| [mark@blommer:~/src/linux]% toolchain korg gcc-8.1.0-nolibc make V=1 ARCH=arm64 CROSS_COMPILE=aarch64-linux- arch/arm64/crypto/sha512-ce.ko | grep aarch64-linux-ld
|   aarch64-linux-ld  -EL  -maarch64elf    -r -o arch/arm64/crypto/sha512-ce.o arch/arm64/crypto/sha512-ce-glue.o arch/arm64/crypto/sha512-ce-core.o
|   aarch64-linux-ld -r  -EL  -maarch64elf  --build-id  -T ./scripts/module-common.lds -T ./arch/arm64/kernel/module.lds -o arch/arm64/crypto/sha512-ce.ko arch/arm64/crypto/sha512-ce.o arch/arm64/crypto/sha512-ce.mod.o;  true

> Maybe you want to tackle that as well? I suggest to have at least one
> FTRACE_CALLSITE_SECTION definition without double quotes. Alternatively, my
> earlier solution just kept both sections, in case either one or both are
> present.
> 
>                         KEEP(*(__patchable_function_entries))   \
>                         KEEP(*(__mcount_loc))                   \

I agree that the CC_USING_PATCHABLE_FUNCTION_ENTRY ifdeffery could be
simplified, and that it would be nice to consistently use
FTRACE_CALLSITE_SECTION if we can. However, the generic linker script doesn't
include anything, and I don't see a good location for that to live.

What I could do is add an explicit comment:

/*
 * The ftrace call sites are logged to a section whose name depends on the
 * compiler option used. A given kernel image will only use one, AKA
 * FTRACE_CALLSITE_SECTION. We capture all of them here to avoid header
 * dependencies.
 */
#define MCOUNT_REC()						\
			. = ALIGN(8);				\
			__start_mcount_loc = .;			\
			KEEP(*(__patchable_function_entries))   \
			KEEP(*(__mcount_loc))                   \
			__stop_mcount_loc = .;			

... which should make the dependency clear. Does that sound good to you?

Thanks,
Mark.

  reply index

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-29 16:58 [PATCHv2 0/8] arm64: ftrace cleanup + FTRACE_WITH_REGS Mark Rutland
2019-10-29 16:58 ` [PATCHv2 1/8] ftrace: add ftrace_init_nop() Mark Rutland
2019-10-30 15:00   ` Miroslav Benes
2019-11-02 12:19   ` Amit Daniel Kachhap
2019-11-04 13:11     ` Steven Rostedt
2019-11-05  6:59       ` Amit Kachhap
2019-11-04 13:36     ` Mark Rutland
2019-11-05  6:47       ` Amit Kachhap
2019-11-06 14:15         ` Mark Rutland
2019-11-07  4:40           ` Amit Kachhap
2019-11-04 13:16   ` Steven Rostedt
2019-11-04 13:38     ` Mark Rutland
2019-11-04 13:53       ` Steven Rostedt
2019-10-29 16:58 ` [PATCHv2 2/8] module/ftrace: handle patchable-function-entry Mark Rutland
2019-10-30 15:03   ` Torsten Duwe
2019-10-31  9:02     ` Mark Rutland [this message]
2019-10-31 11:42       ` Torsten Duwe
2019-10-31 13:00         ` Mark Rutland
2019-11-04 13:28           ` Steven Rostedt
2019-11-04 14:00             ` Mark Rutland
2019-11-04 13:25   ` Steven Rostedt
2019-11-04 15:51   ` Mark Rutland
2019-11-04 20:58     ` Helge Deller
2019-11-05  8:59   ` Miroslav Benes
2019-10-29 16:58 ` [PATCHv2 3/8] arm64: module: rework special section handling Mark Rutland
2019-10-30 15:25   ` Miroslav Benes
2019-10-29 16:58 ` [PATCHv2 4/8] arm64: module/ftrace: intialize PLT at load time Mark Rutland
2019-11-02 12:20   ` Amit Daniel Kachhap
2019-11-04 13:55     ` Mark Rutland
2019-10-29 16:58 ` [PATCHv2 5/8] arm64: insn: add encoder for MOV (register) Mark Rutland
2019-10-29 16:58 ` [PATCHv2 6/8] arm64: asm-offsets: add S_FP Mark Rutland
2019-10-29 16:58 ` [PATCHv2 7/8] arm64: implement ftrace with regs Mark Rutland
2019-11-02 12:21   ` Amit Daniel Kachhap
2019-11-04 13:51     ` Mark Rutland
     [not found]   ` <CANW9uyug8WKN2fR-FmcW-C_OO_OQ_AvukM+BR7wqiJ9eFQMO9Q@mail.gmail.com>
2019-11-15  7:45     ` Torsten Duwe
2019-11-15 13:59     ` Mark Rutland
2019-10-29 16:58 ` [PATCHv2 8/8] arm64: ftrace: minimize ifdeffery Mark Rutland
2019-10-30 17:02 ` [PATCHv2 0/8] arm64: ftrace cleanup + FTRACE_WITH_REGS Torsten Duwe
2019-10-31 17:16 ` Torsten Duwe
2019-11-01  9:08   ` Mark Rutland
2019-11-01 15:39 ` Sven Schnelle
2019-11-01 16:28   ` Mark Rutland
2019-11-02 12:12 ` Amit Daniel Kachhap
2019-11-04 12:56   ` Will Deacon
2019-11-04 13:03     ` Amit Kachhap
2019-11-04 14:04       ` Mark Rutland
2019-11-05  7:06         ` Amit Kachhap
2019-11-07 11:31 ` Catalin Marinas

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