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

[Charlie Jenkins]

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@xxxxxxxxxxxx> 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@xxxxxxxxxx/
> 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.

- Charlie

> 
> Thanks,
> Ian