Re: [PATCH v1 0/7] perf: Support multiple system call tables in the build

Ian Rogers
2025-02-04T19:17:52Z

On Tue, Feb 4, 2025 at 11:05 AM Charlie Jenkins <charlie@rivosinc.com> wrote:
>
> On Mon, Feb 03, 2025 at 05:58:29PM -0800, Ian Rogers wrote:
> > On Mon, Feb 3, 2025 at 3:02 PM Charlie Jenkins <charlie@rivosinc.com> wrote:
> > > On Mon, Feb 03, 2025 at 12:54:59PM -0800, Ian Rogers wrote:
> > [snip]
> > > > I think it makes sense in the kernel, as the built binary doesn't have
> > > > cross-platform concerns. This is probably also the reason why the perf
> > > > tool has an arch directory. Let me know what you think is the right
> > > > direction for the perf tool syscall table code.
> > >
> > > I am hesitant about moving all of the arch-specific syscall generation
> > > flags into a single file.
> >
> > In these changes I had a single file to build up a mapping from ELF
> > machine to syscall table in an array and I wanted to keep the logic to
> > build the array alongside the logic to build up the components of the
> > array - so the ifdefs were visually the same. As the scope is a single
> > file and the variables are static, this can give useful C compiler
> > "unused definition" warnings. You can trick the linker to give similar
> > warnings at the scope of a file, so this isn't a deal breaker.
> >
> > > There is currently a really clean separation
> > > between the architectures and it's possible to generate all of the
> > > syscall tables for all of the architecutures based on the paths to the
> > > syscall table.
> >
> > This doesn't happen currently as the build of the arch directory is to
> > add in $(SRCARCH) only. So
> > tools/perf/arch/arm64/entry/syscalls/Makefile.syscalls will only be
> > included into the build if SRCARCH==arm64. As I've said I'm against
> > the idea of the arch directory as it nearly always causes cross
> > platform problems - not an issue in a Linux kernel build. We recently
> > eliminated dwarf-regs for this reason (and hopefully fixing up cross
> > platform disassembly issues as a consequence):
> > https://lore.kernel.org/all/20241108234606.429459-1-irogers@google.com/
> > We could have the syscall table logic not under arch and generate
> > multiple files, but we'd be adding extra logic to pull things apart to
> > then pull things back together again, which feels like unnecessary
> > complexity.
> >
> > It seems in your changes the Kbuild and Makefile.syscalls are running
> > in the arch directory - this feels like a lot of peculiar and
> > separated build logic for just iterating over a bunch of arch
> > directory names and calling a shell function on them - albeit with
> > some arch specific parameters. There's also an extra C helper
> > executable in your code.
>
> Yes, I can understand why you be opposed to that and I don't have a good
> counterargument.
>
> > I kind of like that I get a single header
> > that is the same across all architectures and with no more build
> > system requirements than to support ifdefs - in fact the ifdefs are
> > just there to keep the code size down there is a #define to make them
> > all have no effect. I hear your "clean separation" but I also think
> > separation across files can make things harder to read, state is in >1
> > place. I've tried to cleanly separate within the script.
> >
> > I do think there is some tech debt in both changes. My:
> > ```
> > #if defined(ALL_SYSCALLTBL) || defined(__riscv)
> > #if __BITS_PER_LONG != 64
> > EOF
> > build_tables "$tools_dir/scripts/syscall.tbl" "$outfile"
> > common,32,riscv,memfd_secret EM_RISCV
> > echo "#else" >> "$outfile"
> > build_tables "$tools_dir/scripts/syscall.tbl" "$outfile"
> > common,64,riscv,rlimit,memfd_secret EM_RISC
> > V
> > cat >> "$outfile" <<EOF
> > #endif //__BITS_PER_LONG != 64
> > ```
> > means the perf binary word size determines the syscall table support.
> > This is because the e_machine in the ELF header isn't unique in these
> > two cases and having both tables would have no effect. You've moved
> > this into the env arch name handling, but I think having >1 way to
> > encode a binary type is suboptimal. There are some ELF flag ABI bits
> > that resolve disassembler things on csky, so perhaps a resolution is
> > to pass ELF flags along with the machine type. I'm not clear in your
> > change how "32_riscv" is generated to solve the same problem. Imo,
> > it'd kind of be nice not to introduce notions like "64_arm64" as we
> > seem to be always mapping/normalizing/... these different notions and
> > they feel inherently brittle.
>
> Maybe it is brittle? It could be mapped to e_machine, but that just
> might not be reasonable to work into the method from my patch.
>
> >
> > > What causes the arch directory to be a pain for Bazel? I
> > > guess I am mostly confused why it makes sense to change the kernel
> > > Makefiles in order to accomidate a rewrite of the build system in Bazel
> > > that isn't planned to be used upstream.
> >
> > It's just software and so the issues are resolvable, ie I don't think
> > bazel should be determining what happens upstream - it motivates me to
> > some extent. For the bazel build I need to match the Makefile behavior
> > with bits of script called genrule, the scope and quantity of these
> > increase with the arch directory model, extra executables to have in
> > the build etc. I also imagine cross platform stuff will add
> > complexity, like mapping blaze's notions of machine type to those
> > introduced in your change. It is all a lot of stuff and I think what's
> > in these changes keeps things about as simple as they can be. It'd be
> > nice to integrate the best features of both changes and I think some
> > of the e_machine stuff here can be added onto your change to get the
> > i386/x86-64 case to work. I'm not sold on the complexity of the build
> > in your changes though, multiple files, Kbuild and Makefile.syscalls,
> > the arch directory not optionally being built, helper executables.
> > Ultimately it is the same shell logic in both changes, and your
> > previous work laid out all the ground work for this (I'm very grateful
> > for it :-) ).
>
> Thanks for elaborating on this! I do wish we had more of this
> conversation on the original patch so we could have molded the original
> patch closer to what this one is doing. I know you did mention you would
> like to get rid of the arch directory as feedback on that patch but I
> hadn't realized that this was the direction you were hoping to take
> this. It does seem like the changes you have made in this patch will
> lead to a something that is more robust and simpler to maintain, so we
> can move forward with reviewing this patch and stop reviewing the one I
> wrote.

Thanks and sorry for work done that may not land - the maintainers may
prefer the approach in your patch and my opinions matter little to
them. If it is any consolation I wrote an entire Rust symbol demangler
[1] that I don't expect to land :-) I also appreciate that you've been
cleaning up the tools build infrastructure along with your changes,
something there isn't enough of.

Thanks,
Ian

[1] https://lore.kernel.org/lkml/20250129193037.573431-1-irogers@google.com/