From: Stefan Agner <stefan@agner.ch>
To: Matthias Kaehlcke <mka@chromium.org>,
Russell King - ARM Linux <linux@armlinux.org.uk>
Cc: ard.biesheuvel@linaro.org, arnd@arndb.de,
nicolas.pitre@linaro.org, marc.zyngier@arm.com,
behanw@converseincode.com, keescook@chromium.org,
Bernhard.Rosenkranzer@linaro.org,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 5/5] ARM: add support for building ARM kernel with clang
Date: Wed, 21 Mar 2018 10:03:22 +0100 [thread overview]
Message-ID: <602b427702af61a503d9ffe38c35075c@agner.ch> (raw)
In-Reply-To: <20180321002022.GH37438@google.com>
On 21.03.2018 01:20, Matthias Kaehlcke wrote:
> El Tue, Mar 20, 2018 at 11:18:33PM +0000 Russell King - ARM Linux ha dit:
>
>> On Wed, Mar 21, 2018 at 12:02:06AM +0100, Stefan Agner wrote:
>> > Use cc-options call for compiler options which are not available
>> > in clang. With this patch an ARMv7 multi platform kernel can be
>> > successfully build using clang (tested with version 5.0.1).
>> >
>> > Based-on-patches-by: Behan Webster <behanw@converseincode.com>
>> > Signed-off-by: Stefan Agner <stefan@agner.ch>
>
> Great to see your work on bringing clang support for 32-bit ARM
> upstream!
>
>> > ---
>> > arch/arm/Makefile | 2 +-
>> > arch/arm/boot/compressed/Makefile | 2 +-
>> > 2 files changed, 2 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/arch/arm/Makefile b/arch/arm/Makefile
>> > index e9e3fde3c657..20e9fee1ccc5 100644
>> > --- a/arch/arm/Makefile
>> > +++ b/arch/arm/Makefile
>> > @@ -39,7 +39,7 @@ KBUILD_CFLAGS += $(call cc-option,-mno-unaligned-access)
>> > endif
>> >
>> > ifeq ($(CONFIG_FRAME_POINTER),y)
>> > -KBUILD_CFLAGS +=-fno-omit-frame-pointer -mapcs -mno-sched-prolog
>> > +KBUILD_CFLAGS +=-fno-omit-frame-pointer $(call cc-option,-mapcs,) $(call cc-option,-mno-sched-prolog,)
>>
>> Some of these options here are to ensure that we generate the following
>> code, so we can backtrace:
>>
>> mov ip, sp
>> stmfd sp!, {fp, ip, lr, pc}
>> sub fp, ip, #4
>>
>> If clang isn't producing that code at the start of functions with
>> CONFIG_FRAME_POINTER=y, then backtracing will not work, and arguably
>> CONFIG_FRAME_POINTER=y is useless there. In that circumstance, it's
>> probably better to fail so the user can configure something more
>> debuggable, rather than having the kernel potentially producing
>> undebuggable oopses.
With clang and -fno-omit-frame-pointer function prologue looks something
like this:
0: e92d4ff0 push {r4, r5, r6, r7, r8, r9, sl, fp, lr}
4: e28db01c add fp, sp, #28
...
This bug seems to be related:
https://bugs.llvm.org/show_bug.cgi?id=18505
It seems that LLVM/clang does not plan to add APCS format/gcc
interoperability for the same frame layout.
I guess it would be possible to support the LLVM/clang frame layout?
But until then, I am with Russel here: Better just let clang fail on
CONFIG_FRAME_POINTER=y.
Most configs probably anyway use CONFIG_ARM_UNWIND. At least
multi_v7_defconfig does.
>
> Which option in particular is important to generate the above code for
> backstracing?
For gcc, I guess really all of them.
--
Stefan
>
> According to the gcc doc -mapcs(-frame) is deprecated.
>
> For -mno-sched-prolog the doc says:
>
> "Prevent the reordering of instructions in the function prologue, or
> the merging of those instruction with the instructions in the
> function’s body. This means that all functions start with a
> recognizable set of instructions (or in fact one of a choice from a
> small set of different function prologues), and this information can
> be used to locate the start of functions inside an executable piece of
> code. The default is -msched-prolog."
>
> Thanks
>
> Matthias
next prev parent reply other threads:[~2018-03-21 9:03 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-03-20 23:02 [PATCH 0/5] ARM: clang support Stefan Agner
2018-03-20 23:02 ` [PATCH 1/5] bus: arm-cci: use asm unreachable Stefan Agner
2018-03-20 23:30 ` Russell King - ARM Linux
2018-03-21 8:22 ` Stefan Agner
2018-03-20 23:02 ` [PATCH 2/5] efi/libstub/arm: add support for building with clang Stefan Agner
2018-03-20 23:02 ` [PATCH 3/5] ARM: trusted_foundations: do not use naked function Stefan Agner
2018-03-20 23:13 ` Russell King - ARM Linux
2018-03-21 8:41 ` Stefan Agner
2018-03-21 12:13 ` Robin Murphy
2018-03-21 14:09 ` Stefan Agner
2018-03-21 15:26 ` Dmitry Osipenko
2018-03-21 16:40 ` Stephen Warren
2018-03-21 17:16 ` Robin Murphy
2018-03-21 21:41 ` Stefan Agner
2018-03-22 11:48 ` Robin Murphy
2018-03-22 12:43 ` Stefan Agner
2018-03-22 14:03 ` Dmitry Osipenko
2018-03-20 23:02 ` [PATCH 4/5] ARM: drop no-thumb-interwork in EABI mode Stefan Agner
2018-03-20 23:02 ` [PATCH 5/5] ARM: add support for building ARM kernel with clang Stefan Agner
2018-03-20 23:18 ` Russell King - ARM Linux
2018-03-21 0:20 ` Matthias Kaehlcke
2018-03-21 9:03 ` Stefan Agner [this message]
2018-03-25 13:24 ` Stefan Agner
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=602b427702af61a503d9ffe38c35075c@agner.ch \
--to=stefan@agner.ch \
--cc=Bernhard.Rosenkranzer@linaro.org \
--cc=ard.biesheuvel@linaro.org \
--cc=arnd@arndb.de \
--cc=behanw@converseincode.com \
--cc=keescook@chromium.org \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux@armlinux.org.uk \
--cc=marc.zyngier@arm.com \
--cc=mka@chromium.org \
--cc=nicolas.pitre@linaro.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).