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Date:   Fri, 17 Jan 2020 10:44:52 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Shuah Khan <shuah@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        linux-rdma@...r.kernel.org,
        "open list\:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH bpf-next v3 02/11] tools/bpf/runqslower: Fix override option for VMLINUX_BTF

Andrii Nakryiko <andrii.nakryiko@...il.com> writes:

> On Thu, Jan 16, 2020 at 5:23 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>>
>> From: Toke Høiland-Jørgensen <toke@...hat.com>
>>
>> The runqslower tool refuses to build without a file to read vmlinux BTF
>> from. The build fails with an error message to override the location by
>> setting the VMLINUX_BTF variable if autodetection fails. However, the
>> Makefile doesn't actually work with that override - the error message is
>> still emitted.
>>
>> Fix this by including the value of VMLINUX_BTF in the expansion, and only
>> emitting the error message if the *result* is empty. Also permit running
>> 'make clean' even though no VMLINUX_BTF is set.
>>
>> Fixes: 9c01546d26d2 ("tools/bpf: Add runqslower tool to tools/bpf")
>> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
>> ---
>>  tools/bpf/runqslower/Makefile |   18 ++++++++++--------
>>  1 file changed, 10 insertions(+), 8 deletions(-)
>>
>> diff --git a/tools/bpf/runqslower/Makefile b/tools/bpf/runqslower/Makefile
>> index cff2fbcd29a8..89fb7cd30f1a 100644
>> --- a/tools/bpf/runqslower/Makefile
>> +++ b/tools/bpf/runqslower/Makefile
>> @@ -10,12 +10,14 @@ CFLAGS := -g -Wall
>>
>>  # Try to detect best kernel BTF source
>>  KERNEL_REL := $(shell uname -r)
>> -ifneq ("$(wildcard /sys/kernel/btf/vmlinux)","")
>> -VMLINUX_BTF := /sys/kernel/btf/vmlinux
>> -else ifneq ("$(wildcard /boot/vmlinux-$(KERNEL_REL))","")
>> -VMLINUX_BTF := /boot/vmlinux-$(KERNEL_REL)
>> -else
>> -$(error "Can't detect kernel BTF, use VMLINUX_BTF to specify it explicitly")
>> +VMLINUX_BTF_PATHS := $(VMLINUX_BTF) /sys/kernel/btf/vmlinux /boot/vmlinux-$(KERNEL_REL)
>> +VMLINUX_BTF_PATH := $(firstword $(wildcard $(VMLINUX_BTF_PATHS)))
>
> If user specifies VMLINUX_BTF pointing to non-existing file, but the
> system has /sys/kernel/btf/vmlinux, the latter will still be used,
> which is a very surprising behavior.

Hmm, yeah, good point.

> Also MAKECMDGOALS feels like a fragile hack to me. How about we move
> this VMLINUX_BTF guessing (without $(error)) into vmlinux.h rule
> itself and use shell if conditional after it to check for file
> existance and print nice error. That way we'll be checking VMLINUX_BTF
> only when it's really needed.

OK, sure, can do.

-Toke

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