Re: [PATCH bpf-next 05/11] samples: bpf: makefile: use D vars from KBUILD_CFLAGS to handle headers

From: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
To: Yonghong Song <yhs@fb.com>
Cc: "ast@kernel.org" <ast@kernel.org>,
	"daniel@iogearbox.net" <daniel@iogearbox.net>,
	"davem@davemloft.net" <davem@davemloft.net>,
	"jakub.kicinski@netronome.com" <jakub.kicinski@netronome.com>,
	"hawk@kernel.org" <hawk@kernel.org>,
	"john.fastabend@gmail.com" <john.fastabend@gmail.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"bpf@vger.kernel.org" <bpf@vger.kernel.org>,
	"clang-built-linux@googlegroups.com" 
	<clang-built-linux@googlegroups.com>
Subject: Re: [PATCH bpf-next 05/11] samples: bpf: makefile: use D vars from KBUILD_CFLAGS to handle headers
Date: Sat, 14 Sep 2019 00:24:04 +0300	[thread overview]
Message-ID: <20190913212402.GB26724@khorivan> (raw)
In-Reply-To: <97ca4228-145a-2449-b4ba-8e79380a54f4@fb.com>

On Fri, Sep 13, 2019 at 09:12:01PM +0000, Yonghong Song wrote:
>
>
>On 9/10/19 11:38 AM, Ivan Khoronzhuk wrote:
>> The kernel headers are reused from samples bpf, and autoconf.h is not
>> enough to reflect complete arch configuration for clang. But CLANG-bpf
>> cmds are sensitive for assembler part taken from linux headers and -D
>> vars, usually used in CFLAGS, should be carefully added for each arch.
>> For that, for CLANG-bpf, lets filter them only for arm arch as it
>> definitely requires __LINUX_ARM_ARCH__ to be set, but ignore for
>> others till it's really needed. For arm, -D__LINUX_ARM_ARCH__ is min
>> version used as instruction set selector. In another case errors
>> like "SMP is not supported" for arm and bunch of other errors are
>> issued resulting to incorrect final object.
>>
>> Later D_OPTIONS can be used for gcc part.
>> ---
>>   samples/bpf/Makefile | 9 +++++++++
>>   1 file changed, 9 insertions(+)
>>
>> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
>> index 8ecc5d0c2d5b..6492b7e65c08 100644
>> --- a/samples/bpf/Makefile
>> +++ b/samples/bpf/Makefile
>> @@ -185,6 +185,15 @@ HOSTLDLIBS_map_perf_test	+= -lrt
>>   HOSTLDLIBS_test_overhead	+= -lrt
>>   HOSTLDLIBS_xdpsock		+= -pthread
>>
>> +# Strip all expet -D options needed to handle linux headers
>> +# for arm it's __LINUX_ARM_ARCH__ and potentially others fork vars
>> +D_OPTIONS = $(shell echo "$(KBUILD_CFLAGS) " | sed 's/[[:blank:]]/\n/g' | \
>> +	sed '/^-D/!d' | tr '\n' ' ')
>> +
>> +ifeq ($(ARCH), arm)
>> +CLANG_EXTRA_CFLAGS := $(D_OPTIONS)
>> +endif
>
>Do you need this for native compilation?
Yes, native "arm" also requires it.

>
>so arm64 compilation does not need this?
yes, now only arm

>If only -D__LINUX_ARM_ARCH__ is needed, maybe just
>with
>    CLANG_EXTRA_CFLAGS := -D__LINUX_ARM_ARCH__
Value also needed: -D__LINUX_ARM_ARCH_=7 or -D__LINUX_ARM_ARCH_=6
So, need retrieve it.

>Otherwise, people will wonder whether this is needed for
>other architectures. Or just do
>    CLANG_EXTRA_CFLAGS := $(D_OPTIONS)
>for all cross compilation?
arm, cross and native requires it.

Will do this:

# Strip all expet -D options needed to handle linux headers
# for arm it's __LINUX_ARM_ARCH__ and potentially others fork vars
ifeq ($(ARCH), arm)
D_OPTIONS = $(shell echo "$(KBUILD_CFLAGS) " | sed 's/[[:blank:]]/\n/g' | \
	sed '/^-D/!d' | tr '\n' ' ')
endif

CLANG_EXTRA_CFLAGS := $(D_OPTIONS)

>
>> +
>>   # Allows pointing LLC/CLANG to a LLVM backend with bpf support, redefine on cmdline:
>>   #  make samples/bpf/ LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang
>>   LLC ?= llc
>>

-- 
Regards,
Ivan Khoronzhuk