Re: [PATCH 2/2] tools: perf: tests: Fix code reading for riscv

[Charlie Jenkins <charlie@rivosinc.com>]

On Mon, Dec 16, 2024 at 08:57:20PM -0800, Ian Rogers wrote:
> On Mon, Dec 16, 2024 at 3:13 PM Charlie Jenkins <charlie@rivosinc.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@rivosinc.com>
> > ---
> >  arch/riscv/Kconfig              |  5 +++++
> >  tools/perf/tests/code-reading.c | 17 ++++++++++++++++-
> 
> Files under tools use a different Build system than the kernel. The
> Kconfig value won't have an effect. Check out Makefile.config:
> https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/Makefile.config?h=perf-tools-next
> which is included into the build here:
> https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/Makefile.perf?h=perf-tools-next#n313
> 

Ahh okay, thank you. It was properly enabling when I was testing, is
there some bleeding over?

> >  2 files changed, 21 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index d4a7ca0388c071b536df59c0eb11d55f9080c7cd..f164047471267936bc62389b7d7d9a7cbdca8f97 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -229,6 +229,11 @@ config GCC_SUPPORTS_DYNAMIC_FTRACE
> >         def_bool CC_IS_GCC
> >         depends on $(cc-option,-fpatchable-function-entry=8)
> >
> > +config RISCV_OBJDUMP_SUPPORTS_SPLIT_INSTRUCTION
> > +       # Some versions of objdump do not support dumping partial instructions
> > +       def_bool y
> > +       depends on !(OBJDUMP_IS_GNU && OBJDUMP_VERSION > 24100)
> > +
> >  config HAVE_SHADOW_CALL_STACK
> >         def_bool $(cc-option,-fsanitize=shadow-call-stack)
> >         # https://github.com/riscv-non-isa/riscv-elf-psabi-doc/commit/a484e843e6eeb51f0cb7b8819e50da6d2444d769
> > diff --git a/tools/perf/tests/code-reading.c b/tools/perf/tests/code-reading.c
> > index 27c82cfb7e7de42284bf5af9cf7594a3a963052e..605f4a8e1dbc00d8a572503f45053c2f30ad19e3 100644
> > --- a/tools/perf/tests/code-reading.c
> > +++ b/tools/perf/tests/code-reading.c
> > @@ -1,5 +1,6 @@
> >  // SPDX-License-Identifier: GPL-2.0
> >  #include <errno.h>
> > +#include <linux/kconfig.h>
> >  #include <linux/kernel.h>
> >  #include <linux/types.h>
> >  #include <inttypes.h>
> > @@ -183,9 +184,23 @@ 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) && !IS_ENABLED(CONFIG_RISCV_OBJDUMP_SUPPORTS_SPLIT_INSTRUCTION)) {
> 
> It would be nice if this could be a runtime rather than build time detected.

Hmm that is a good point. I will change this to check the version at
runtime.

- Charlie

> 
> Thanks,
> Ian
> 
> > +               /*
> > +                * On some versions of riscv objdump, dumping in the middle of
> > +                * instructions is not supported. riscv instructions are aligned along
> > +                * 2-byte intervals and can be either 2-bytes or 4-bytes. This makes it
> > +                * possible that the stop-address lands in the middle of a 4-byte
> > +                * instruction. Increase the stop_address by two to ensure an
> > +                * instruction is not cut in half, but leave the len as-is so only the
> > +                * expected number of bytes are collected.
> > +                */
> > +               stop_address += 2;
> > +       }
> >
> >         fmt = "%s -z -d --start-address=0x%"PRIx64" --stop-address=0x%"PRIx64" %s";
> > -       ret = snprintf(cmd, sizeof(cmd), fmt, test_objdump_path, addr, addr + len,
> > +       ret = snprintf(cmd, sizeof(cmd), fmt, test_objdump_path, addr, stop_address,
> >                        filename);
> >         if (ret <= 0 || (size_t)ret >= sizeof(cmd))
> >                 return -1;
> >
> > --
> > 2.34.1
> >