Message-ID: <CAP-5=fUBcu=Kd7StJvwdM_ArnSdOjmAcgR7NzDd+Ep8LrNMhyw@mail.gmail.com>
Date: Wed, 18 Dec 2024 17:20:15 -0800
From: Ian Rogers <irogers@...gle.com>
To: Charlie Jenkins <charlie@...osinc.com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...hat.com>, Namhyung Kim <namhyung@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Adrian Hunter <adrian.hunter@...el.com>, Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v2] tools: perf: tests: Fix code reading for riscv

On Wed, Dec 18, 2024 at 2:32 PM Charlie Jenkins <charlie@...osinc.com> wrote:
>
> On Wed, Dec 18, 2024 at 02:13:20PM -0800, Ian Rogers wrote:
> > On Wed, Dec 18, 2024 at 1:02 PM Charlie Jenkins <charlie@...osinc.com> wrote:
> > >
> > > On Wed, Dec 18, 2024 at 11:23:51AM -0800, Ian Rogers wrote:
> > > > On Wed, Dec 18, 2024 at 10:41 AM Arnaldo Carvalho de Melo
> > > > <acme@...nel.org> wrote:
> > > > >
> > > > > On Tue, Dec 17, 2024 at 04:30:15PM -0800, Charlie Jenkins wrote:
> > > > > > On Tue, Dec 17, 2024 at 04:18:32PM -0800, Ian Rogers wrote:
> > > > > > > On Tue, Dec 17, 2024 at 3:52 PM Charlie Jenkins <charlie@...osinc.com> wrote:
> > > > > > > > After binutils commit e43d876 which was first included in binutils 2.41,
> > > > > > > > riscv no longer supports dumping in the middle of instructions. Increase
> > > > > > > > the objdump window by 2-bytes to ensure that any instruction that sits
> > > > > > > > on the boundary of the specified stop-address is not cut in half.
> > > > >
> > > > > > > > Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
> > > > >
> > > > > > > Reviewed-by: Ian Rogers <irogers@...gle.com>
> > > > >
> > > > > > > > A binutils patch has been sent as well to fix this in objdump [1].
> > > > >
> > > > > > > > Link: https://sourceware.org/pipermail/binutils/2024-December/138139.html [1]
> > > > >
> > > > > > > > Changes in v2:
> > > > > > > > - Do objdump version detection at runtime (Ian)
> > > > > > > > - Link to v1: https://lore.kernel.org/r/20241216-perf_fix_riscv_obj_reading-v1-0-b75962660a9b@rivosinc.com
> > > > >
> > > > > > > > --- a/tools/perf/tests/code-reading.c
> > > > > > > > @@ -183,9 +244,30 @@ static int read_via_objdump(const char *filename, u64 addr, void *buf,
> > > > > > > >         const char *fmt;
> > > > > > > >         FILE *f;
> > > > > > > >         int ret;
> > > > > > > > +       u64 stop_address = addr + len;
> > > > > > > > +
> > > > > > > > +       if (IS_ENABLED(__riscv)) {
> > > > >
> > > > > > > Not sure if there is a consistency issue here. Elsewhere we're just
> > > > > > > using ifdef, such as:
> > > > > > > https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/include/dwarf-regs.h?h=perf-tools-next#n69
> > > > >
> > > > > > I don't have any strong feelings about that. I can change it to be an
> > > > > > ifdef. On other lists I have been told to use IS_ENABLED whenever
> > > > > > possible, but it's only a small difference.
> > > > >
> > > > > Can't we just use uname here?
> > > > >
> > > > > So that we don't use kconfig.h since its not used in tools/perf/ and
> > > > > makes it looks like perf is in lockstep with the kernel source tree
> > > > > version it was compiled from?
> > > > >
> > > > > $ git grep kconfig.h tools/perf/
> > > > > $
> > > > >
> > > > > BTW, what would happen if I collected a perf.data file on x86_64 and
> > > > > would read it in a RiscV machine with such a objdump version? The same
> > > > > problem?
> > > >
> > > > This code is in tests hence thinking that a separate fix is needed for
> > > > that problem. Hopefully the use of elf machine/flags tackles it:
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/include/dwarf-regs.h?h=perf-tools-next#n25
> > > > We are getting somewhat disassembler heavy. We have llvm as a library,
> > > > capstone as a library, binutils objdump and llvm objdump. Given the
> > > > pain with parsing text, could we lose the objdumps? Similarly for
> > > > addr2line?
> > >
> > > Are you suggesting to remove this test case entirely to get rid of the
> > > objdump dependency? The goal of this test case seems to be to check
> > > objdump and perf return the same data, so it doesn't seem like there
> > > would be an alternative to using objdump.
> >
> > I can imagine having an objdump dependency for a test but not for some
> > more core like `perf annotate`. We have to do weird things when
> > parsing text, like this code I'm not proud of:
> > https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/srcline.c?h=perf-tools-next#n523
> > The issue with that code is that LLVM objdump has changed its output
> > in newer versions to be closer to binutils objdump. Did that break
> > perf? Maybe it just broke what our variables think is an LLVM objdump,
> > but things aren't really broken. This kind of issue doesn't occur with
> > a library, although the differing needs of library versions is a real
> > thing.
>
> Yeah doing the parsing of the text output is not ideal... For this test
> case it should be possible to dynamically link against libbfd.

I need to write the patch set to delete libbfd from perf. IANAL but
the issue is that libbfd is part of binutils and GPLv3, while perf is
part of the Linux kernel and largely GPLv2. GPLv3 is incompatible with
GPLv2:
https://www.gnu.org/licenses/gpl-faq.html#AllCompatibility
While using dlopen means we're not linking against libbfd, we may
effectively be using it as a plugin which again GPLv3 (in my IANAL
opinion) wouldn't allow:
https://www.gnu.org/licenses/gpl-faq.en.html#GPLPlugins
Currently to get libbfd support in perf you need to be building the
binary yourself and add to the build BUILD_NONDISTRO=1. We do this as
part of our build testing but having all the #ifdef-ed libbfd code if
nothing else makes the code harder to understand.

> I would
> guess something similar could be done with llvm-objdump but I am less
> familiar with that. I don't know if that's a good path to go down
> though.

In the past I perceived there was hostility toward LLVM from the Linux
kernel community. I guess GPL was considered the special sauce as to
why Linux won and the BSDs hadn't, so the preference was to favor a
compiler that used the same license. I don't think that's true any
more and I think there's a lot of sense in using LLVM's libraries
rather than reinventing them in the perf tool, or using perhaps less
orthodox sources like libcapstone. I'm not a fan of the text output
processing stuff so getting rid of objdump and llvm-objdump support
would be good imo.

Another area where I think we could lose a lot of code baggage is with
libunwind, as BPF support requires libelf which brings with it
libdwarves which when present means we don't use libunwind. I've heard
reports that libdwarves is slower, but I'm sure we can add caches to
speed it up which would likely benefit a range of people. I was kind
of hoping with all that deleted we may be able to get rid of the
majority of the arch directory, but the syscalltbl work is adding to
that directory :-)

Thanks,
Ian

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