linux-kernel - Re: [tip: objtool/core] x86/insn: Support big endian cross-compiles

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Date:   Tue, 3 Nov 2020 20:35:00 +0100
From:   Vasily Gorbik <gor@...ux.ibm.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        x86 <x86@...nel.org>
Subject: Re: [tip: objtool/core] x86/insn: Support big endian cross-compiles

On Wed, Oct 14, 2020 at 04:28:59PM +0900, Masami Hiramatsu wrote:
> On Mon, 12 Oct 2020 10:39:49 -0500
> Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
> > On Mon, Oct 12, 2020 at 09:12:36AM +0900, Masami Hiramatsu wrote:
> > > On Sat, 10 Oct 2020 12:44:15 -0500
> > > Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > > 
> > > > On Fri, Oct 09, 2020 at 10:49:21PM +0200, Borislav Petkov wrote:
> > > > > On Fri, Oct 09, 2020 at 10:38:22PM +0200, Peter Zijlstra wrote:
> > > > > > On Wed, Oct 07, 2020 at 04:20:19PM -0000, tip-bot2 for Martin Schwidefsky wrote:
> > > > > > > The following commit has been merged into the objtool/core branch of tip:
> > > > > > > 
> > > > > > > Commit-ID:     2a522b53c47051d3bf98748418f4f8e5f20d2c04
> > > > > > > Gitweb:        https://git.kernel.org/tip/2a522b53c47051d3bf98748418f4f8e5f20d2c04
> > > > > > > Author:        Martin Schwidefsky <schwidefsky@...ibm.com>
> > > > > > > AuthorDate:    Mon, 05 Oct 2020 17:50:31 +02:00
> > > > > > > Committer:     Josh Poimboeuf <jpoimboe@...hat.com>
> > > > > > > CommitterDate: Tue, 06 Oct 2020 09:32:29 -05:00
> > > > > > > 
> > > > > > > x86/insn: Support big endian cross-compiles
> > > > > > > 
> > > > > > > x86 instruction decoder code is shared across the kernel source and the
> > > > > > > tools. Currently objtool seems to be the only tool from build tools needed
> > > > > > > which breaks x86 cross compilation on big endian systems. Make the x86
> > > > > > > instruction decoder build host endianness agnostic to support x86 cross
> > > > > > > compilation and enable objtool to implement endianness awareness for
> > > > > > > big endian architectures support.
> > > > > > > 
> > > > > > > Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
> > > > > > > Co-developed-by: Vasily Gorbik <gor@...ux.ibm.com>
> > > > > > > Signed-off-by: Vasily Gorbik <gor@...ux.ibm.com>
> > > > > > > Acked-by: Masami Hiramatsu <mhiramat@...nel.org>
> > > > > > > Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > > > > > 
> > > > > > This commit breaks the x86 build with CONFIG_X86_DECODER_SELFTEST=y.
> > > > > > 
> > > > > > I've asked Boris to truncate tip/objtool/core.
> > > > > 
> > > > > Yeah, top 4 are gone until this is resolved.
> > > > 
> > > > Masami, I wonder if we even need these selftests anymore?  Objtool
> > > > already decodes the entire kernel.
> > > 
> > > No, they have different roles. The selftest checks if the decoder
> > > works correctly by comparing with the output of objdump.
> > > 
> > > As far as I can see, the objtool relies on the sanity of the decoder
> > > (it trusts the output of the decoder).
> > 
> > Ok.  I wonder if we should move the decoder selftest to the 'tools'
> > subdirectory.
> 
> It is in the arch/x86/tools, so it is already in a kind of tools :)
> But yeah, it was considered to be used only on x86. But if someone
> start trying to run it on non-x86, cross compiling, we need to
> reconsider that.

I actually tried to move it to tools/testing/selftests and encountered
several problems with kselftest build in general:
- out of source build is broken if path is relative,
- out of source build headers partially installed in
  $(srcdir)arch/x86/include/generated/ instead of $(objdir), when
  kselftests are called from the kbuild,
- out of source test runs is broken,
- kernel headers are installed unconditionally.

These things impede moving decoder selftests to kselftests.

On the other hand making the decoder selftest work "in place" seems
trivial. The following fix on top of jpoimboe/objtool/core fixes the
build, as well as cross-compilation. With that I can cross-compile
x86 kernel on s390 with CONFIG_X86_DECODER_SELFTEST=y and posttest runs
just fine.

Vasily Gorbik (1):
  x86/tools: Use tools headers for instruction decoder selftests

 arch/x86/tools/Makefile      | 8 ++++----
 arch/x86/tools/insn_sanity.c | 4 ----
 2 files changed, 4 insertions(+), 8 deletions(-)

-- 
2.25.4