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
